Stamp Collectors: growing
a stamp marketplace with auctions
and eBay integration
Stamp Collectors is a specialized postage stamp marketplace built on CS-Cart Multi-Vendor. Users buy and sell stamps at fixed prices, bid in auctions and make offers to sellers.
Industry: philately Country: US Website: shop.stampcollectors.us
rows in the feed — the entire catalog approved by Google Merchant Center
search response instead of almost a minute (client’s measurements, September 2026)
app installs on Google Play
devices subscribed to push notifications
ABOUT THE CLIENT
Stamp Collectors combines a marketplace, a community and reference materials for collectors. The platform was founded by a collector, the author of the philately YouTube channel That Stamp Guy. He trades stamps himself, so the platform is built by collectors, for collectors.
The client's challenge
The marketplace needed new features and more automation. The Multi-Vendor platform with three vendor plans was already running, but sellers added products manually, and vendor plan payments had to be tracked by hand. There was no eBay integration, no auctions, no automatic payments and no mobile app.
The key difficulty was the nature of collectible stamps: most lots exist in a single copy. Most stamps are one of a kind, so without stock sync between eBay and the marketplace, the same item could sell twice. Without stock synchronization between eBay and the marketplace, the same item could be sold twice. At the same time, many sellers used personal PayPal accounts, so buyers had to pay sellers directly, with the platform fee deducted automatically.
To grow the marketplace, the client also needed to bring in sellers from eBay without manual catalog transfer, list products on Google Shopping and provide the mechanics buyers are used to: search by catalog number, auctions, bids and price negotiation.
Main tasks
- Build an eBay integration: product import, category and feature mapping, price and stock updates.
- Improve the auction lifecycle: launch, editing, auto-restart and closing.
- Add price negotiation via Best Offer.
- Automate vendor subscription payments and PayPal account connection.
- Bring buying and selling management together in the Vendor Panel (Dashboard).
- Adapt search, the catalog and vendor microstores to the platform’s specifics.
- Simplify shipping setup and update the USPS integration.
- Add seller control tools and automatic data transfer to external services.
- Release a mobile app for buyers and sellers with a full-featured vendor panel.
What We Did
INTEGRATION
eBay Integration: One Catalog, Two Marketplaces
Many stamp sellers already have thousands of listings on eBay. Re-entering them by hand would take weeks, and keeping stock in sync manually is risky: most stamps are one of a kind, so a single mistake means selling the same stamp twice.
We built the eBay Product Sync add-on. Sellers connect their eBay store, and their listings appear on Stamp Collectors automatically, with stock kept in sync on both sides.
Two Ways to Connect
Basic Mode
The seller enters their eBay seller name and picks the items to import. No eBay developer account needed. Stamp Collectors regularly checks that imported items are still available.
Advanced Mode
The seller connects with their own eBay API keys. This adds export to eBay and two-way stock sync. Each vendor plan includes one of three options: no sync, Basic or Advanced.
No Re-entering Data
Items arrive with their title, description, photos, price, specifics and the seller’s own SKU. eBay categories are matched to the Stamp Collectors catalog automatically, so sellers don’t have to sort stamps by hand. Items with no match go to a default category (e.g. Stamps › Worldwide › Other Worldwide Stamps).
Product details are copied once, when the item is first imported. After that, only price and stock are updated, so the seller’s edits on Stamp Collectors are never overwritten.
Automatic Import
Step 1
Checking eBay for new products
Step 2
Adding items to the import list
Step 3
Creating products on Stamp Collectors
Sellers can run the whole process automatically or review items before import. In Basic mode, only fixed-price listings are imported, not auctions. Synchronization can be disabled for individual products.
Large Catalogs, Same One Button
eBay limits how many requests an app can make per day, so a big catalog couldn’t be moved in one go. We reworked the sync to use fewer, larger requests and run it through several eBay developer accounts that switch automatically when one hits its limit. For the seller, it’s still one button.
Stock Stays in Sync
For linked products, we set up handling of stock and availability changes. This two-way sync is available with the Advanced connection.
Stamp Collectors → eBay
Instantly
A sale or stock change updates eBay right away.
eBay → Stamp Collectors
Within an hour
Sales, stock and price changes on eBay are updated in the store by a cron command.
AUCTIONS
Auctions: from Launch to Close
We combined the improvements into one consistent flow: the seller creates a lot, starts bidding, gets bid notifications and decides what happens to an unsold item.
Launch and Editing
The seller turns a Buy It Now lot into an auction and chooses its duration, just like on eBay. The bid increment is calculated dynamically from the current price. Auctions can be cloned.
If all required fields are set and the start time has passed, the auction goes live immediately.
Sellers can edit product details right on the auction page — until the first bid is placed.
Auction Auto-Restart
For auctions without a winner, we added auto-restart. The system updates the start time and calculates a new end time while keeping the original duration. A log entry is created.
The auction repeats until there is a winner, the restart checkbox is unchecked, or the auction is finished manually. If the linked product is deleted or disabled, the auction closes automatically at the restart point.
Bids, Timers and Bidding History
Sellers get a New Bid Received notification with the product name, bid amount and auction link. We redesigned the countdown timers and the end date display, as on eBay. The storefront got an Auctions ending soon block, and product lists now show the number of bids.
- Auction ends
- Day 1
- Day 2
- Day 5
Ended auctions and sold products are hidden from regular listings but remain accessible through the Show Ended Auctions and Show Sold Products filters and direct links from email notifications. For a sold product, instead of a 404 error, a message says it has already been purchased.
NEGOTIATION
Best Offer: Negotiating an Individual Price
In addition to fixed prices and auctions, a buyer can offer the seller their own price right from the product page.
01
The buyer makes an offer
A request from the product page
02
The seller responds
Accepts, declines or makes a counteroffer
03
Order at the agreed price
Once approved, the buyer places the order
The feature includes:
- Best Offer for the entire catalog or individual products;
- bulk setup via import and export;
- notifications about new offers and status changes;
- linking an offer to the placed order;
- communication via the Message Center add-on.
PAYMENTS
Vendor Plans and PayPal: Registration, Subscriptions and Receiving Payments
We separated two processes: the vendor paying for their plan and connecting PayPal to receive money from buyers. Each has its own logic tied to registration and vendor permissions.
Automation of vendor subscription payments
Cart-Power: PayPal Subscriptions add-on: the administrator creates PayPal Subscription Plans for monthly and yearly vendor plans and manages their availability. When registering on a paid plan, the user subscribes right away.
Transactions are stored in billing, API calls in the log. Failed payments are handled, and vendors are suspended based on the payment failure threshold, balance and grace period.
Personal and business accounts
We modified the PayPal Commerce Platform add-on to support both account types. The buyer pays the seller directly, and the platform’s commission is deducted automatically.
Personal accounts required a separate onboarding flow and setting up the platform’s account together with PayPal. Both the seller and the administrator can see the connection status and PayPal account ID.
Different Rules for Paid and Free Vendor Plans
On a paid plan, PayPal onboarding is a required registration step. On the free plan, the user can explore the dashboard first and is redirected to connect PayPal when trying to create a product or sync with eBay. If a required step isn’t completed, the system brings the user back to it.
Switching to the Starter Plan without Deleting the Catalog
After a subscription is canceled, the vendor plan stays unchanged until the end of the paid period, then the account is automatically moved to the Starter plan with the Active status. If there are more than 300 products, the latest 300 stay active and the rest are hidden/disabled but not deleted. Products on eBay remain active.
DASHBOARD
One dashboard for Buying and Selling
Every registration on the website and in the app is a vendor registration on the free plan, and the same account can start buying right away. Previously, orders were managed in two places, which confused users. We moved all order, auction and offer management to the Vendor Panel (Dashboard), leaving the cart and checkout on the storefront.
As a buyer
My Purchases — orders, including auction purchases;
My Best Offers — sent offers;
My Bids — placed bids.
As a seller
Received Offers — offers received;
Auctions — auctions for their products.
SHIPPING
Shipping: USPS Setup and Cost Information Before Checkout
We adjusted the existing USPS integration add-on for the new API requirements: updated the integration, copied the service and settings template, configured a new shipping method and tested rate receiving. Both domestic and international shipments are supported.
All shipping methods created by the marketplace administrator are automatically assigned to vendors: USPS First Class is enabled by default, the rest are disabled. Vendors can enable or disable the methods they need in the vendor panel or create a Custom Shipping Rate. This replaced the idea of importing shipping rules from eBay — it’s simpler and doesn’t get in the way of updates.
Cart-Power: Minimum Shipping Cost Add-on
Shows “+ N shipping” or “Free shipping” in product lists, depending on product, vendor and add-on settings. The buyer can estimate the total in advance, while the add-on is informational only and does not affect the actual shipping calculation.
CATALOG
Catalog, Search and Moderation
Navigation is organized by country: USA, Canada, Great Britain, British Colonies, Europe, Asia and Worldwide. Product pages show a Products and auctions from the same category block, and the vendor logo links to the vendor’s microstore.
Collectors search for stamps by catalog number, so search by product names and feature values takes exact number matches into account: the query “573” finds “573”, “573.” and “#573”, but not “5731” or “$573”. Search works across the site, within a category (Search Within Category) and within a seller’s store (Search This Seller).
✓ 573.
✓ #573
✕ 5731
✕ $573
A catalog of hundreds of thousands of lots. We sped up search with a separate cache table.
Automatic Moderation
- non-stamp lots get the new Invalid status, which hides them from the storefront;
- photos are required, and a zero price is not allowed;
- links, emails and images are removed from descriptions.
EXTERNAL SERVICES
Sending Data to Google Sheets and Merchant Center
Vendor to Sheet add-on
When a seller registers, their ID, name, email and vendor plan are automatically sent to a Google Sheet. After PayPal is connected, the PayPal ID is added; after eBay is connected — a corresponding flag (Y).
Data on sellers’ readiness to start selling is collected in one place without manual entry.
Feed for Google Merchant Center
We modified the default CS-Cart add-on Export Product Data to Google Merchant Center: added shipping and returns data, scheduled feed generation via cron and automatic submission to Merchant Center.
209,000+ rows, the entire catalog approved
IOS AND ANDROID
Mobile App
The app is built on Cart-Power PowerApp (React Native) and works through the CS-Cart API. All the logic of the web version has been carried over, including registration based on the vendor plan and connecting PayPal and eBay. It was published under the client’s accounts in early 2026.
For buyers
Catalog, auctions and bids, Make an Offer, PayPal payments, push notifications, sign-in with Google and Facebook.
For vendors
A full-featured vendor panel: dashboard, creating and editing lots with features, orders, auctions, responding to price offers.
Results
Within the project, we connected the key processes of a specialized marketplace:
- importing and updating inventory from eBay;
- selling at a fixed price, through auctions and via negotiated offers;
- vendor registration, PayPal account connection and vendor plan payments;
- managing purchases and sales from the Vendor Panel (Dashboard);
- search by product names and feature values with defined matching rules;
- shipping setup and showing estimated shipping costs upfront;
- vendor control and automatic data transfer to external services.
The main result is consistent process logic: turning a product into an auction is taken into account during eBay synchronization, the end of a subscription changes the active inventory without deleting products, and one account manages both sides of a deal.
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