Marketplace Integration for Retailers: Connecting Amazon, eBay and Your Own Store
How retailers connect Amazon, eBay and their own store: listings and product data, stock sync, orders and fulfilment, returns, fees and UK VAT on marketplaces.
How should retailers integrate Amazon, eBay and their own store?
Keep one source of truth for products and stock (your ecommerce platform, a PIM, an inventory system or an ERP), publish listings and availability to Amazon and eBay from it, pull marketplace orders back into one order flow for fulfilment, and reconcile marketplace payouts, fees and refunds in accounting. Most retailers start with a multichannel tool; custom integration earns its place when catalogues, fulfilment or systems are complex.
This guide is for ecommerce managers and technical leads at retailers selling on their own store and marketplaces.
Key takeaways
- Decide the source of truth for products, stock and orders.
- Map product data to each marketplace's categories and attributes.
- Sync stock on every change, with buffers for fast sellers.
- Bring all orders into one fulfilment flow.
- Reconcile payouts, fees and refunds, not just orders.
Integration architecture
| Flow | Direction | Key points |
|---|---|---|
| Product data and listings | Source of truth → marketplaces | Category mapping, required attributes, identifiers (GTINs), images, variations |
| Prices and offers | Source → marketplaces | Channel-specific pricing, fees, promotions |
| Stock availability | Inventory → marketplaces | Frequent sync, buffers, reservations |
| Orders | Marketplaces → order management | Notifications or polling; de-duplication; customer data handling |
| Fulfilment and tracking | Order management → marketplaces | Ship confirmations and tracking within marketplace deadlines |
| Returns and refunds | Both ways | Marketplace return rules; stock and refund updates |
| Settlements | Marketplaces → accounting | Payout reports, fees, adjustments |
Marketplace APIs
Amazon's Selling Partner API provides APIs for listings, orders, inventory, reports and notifications, with seller authorisation and rate limits per operation. eBay's developer programme provides Sell APIs for inventory, offers and fulfilment. Both change over time, so isolate each marketplace behind an adapter and monitor deprecations. For Amazon fulfilment (FBA), stock held in Amazon's network needs separate tracking from your own warehouse stock.
Product data is the hard part
Each marketplace has its own categories, required attributes, variation rules and content limits. Rejected or suppressed listings usually trace back to missing attributes or identifiers. Keep rich product data in one place, with channel-specific mappings and overrides, rather than editing listings by hand. See product information management.
Stock sync and overselling
- Push availability on every stock change, plus a periodic full sync to catch drift.
- Reserve stock as soon as any channel's order arrives.
- Use per-channel buffers for fast-moving or low-stock items.
- Handle bundles and kits by computing availability from components.
- Alert when a channel's listed quantity diverges from your system.
UK VAT note
Since 1 January 2021, online marketplaces have been responsible for VAT on goods sold to UK consumers by overseas sellers where the goods are in the UK at the point of sale, and on imported consignments of £135 or less (GOV.UK policy paper). Northern Ireland has different rules for some goods. UK-established sellers generally account for their own VAT. Your integration should record which party accounted for VAT on each order so accounting is correct. Confirm the treatment with your accountant.
Hypothetical example: a homeware retailer
This example is hypothetical. A UK homeware retailer selling on Shopify, Amazon and eBay oversold popular items during promotions because stock was updated hourly from a spreadsheet. It made Shopify the source of truth for products and stock, connected Amazon and eBay through a multichannel tool, added component-based availability for gift sets with a small custom service, and imported settlement reports into its accounting system weekly. Oversells became rare and marketplace fees became visible by product.
Common mistakes
- Editing listings by hand in each marketplace.
- Slow or batch-only stock sync.
- Orders processed in each marketplace separately.
- Ignoring fees and adjustments in reconciliation.
- No monitoring of API changes and suppressed listings.
Sources
Amazon Selling Partner API documentation; SP-API Orders API; eBay Developers Program; GOV.UK: VAT treatment of overseas goods sold to customers.
Accessed 10 October 2026. Nothing here is tax advice.
Conclusion
Marketplace integration works when products and stock have one source of truth, product data is mapped once per channel, stock syncs on every change, orders flow into one fulfilment process and payouts are reconciled. Start with a multichannel tool and add custom services where your catalogue or systems need them. See omnichannel ecommerce architecture and payment reconciliation automation.
Overselling or re-keying marketplace orders?
ZSpace Labs is a remote-first technology studio that works with clients globally on Shopify builds and integrations and custom commerce integrations. If it would help to map your channels and systems, we are happy to talk it through.
Common questions.
Start with an established multichannel listing or inventory tool if it supports your channels, catalogue and fulfilment. Custom integration makes sense when you have unusual product data, bundles or kits, several warehouses, an ERP that must stay the source of truth, or volumes where off-the-shelf tools struggle.