Inventory sync, usually called stock sync in the UK, is the automatic updating of stock levels across every place you sell, so that when an item sells on one channel the available quantity drops everywhere else. It keeps your website, marketplaces, physical shop and product feeds showing the same, accurate stock.
How inventory sync works
One system holds the master record of stock: usually your ecommerce platform, an order management system, a stock control tool or an ERP. Every channel connects to it:
- your website, if it is not the master itself;
- marketplaces such as Amazon, eBay, Etsy or OnBuy;
- the EPOS system in a physical shop;
- your warehouse or 3PL, which reports goods received and dispatched;
- product feeds for Google Merchant Center and Meta, which carry availability.
When stock changes in one place, the change is pushed to the others. Some connections update almost instantly using webhooks; others check every few minutes or once an hour. The gap between updates is when overselling happens: two people buy the last item on two different channels before either channel knows about the other sale.
To cover that gap, many sellers hold a stock buffer, keeping one or two units back from marketplaces so a popular item shows as sold out there just before it actually runs out. Products are matched across channels by SKU, so consistent SKUs are the foundation of any sync.
Why it matters for a UK business
Overselling creates cancelled orders, apologetic emails and refunds. On marketplaces the damage is greater, because cancellation rates count towards seller performance measures and can lead to restrictions. Selling on an online marketplace without reliable sync is one of the quickest ways for a small seller to get into trouble.
The reverse problem costs money too. If sync fails and products show as out of stock when you have them on the shelf, you lose sales without noticing.
Advertising depends on it. Google Shopping and Meta catalogue ads read availability from your feeds. If an item has sold out but the feed still says “in stock”, you pay for clicks that cannot convert, and Google may disapprove the product for an availability mismatch. With accurate data, ads stop serving during stockouts on their own.
Common mistakes
- More than one master. If stock is edited by hand in two systems, they will drift apart.
- Inconsistent SKUs between the website, marketplaces and warehouse, so the sync tool cannot match products.
- Infrequent feed updates. A product feed refreshed once a day will advertise items that sold out that morning.
- No buffer on fast sellers. High-demand lines with slow sync are where overselling concentrates.
- Forgetting bundles. A bundle made of three products has to draw stock from all three, which basic sync tools often miss.
How to act on it
Decide which system is the single source of truth for stock, and make sure nobody edits quantities anywhere else. Audit your SKUs so every product and variant has one code used on every channel.
Check how often each connection updates. If any channel lags by more than a few minutes on popular lines, set a buffer or move to a tool that supports near-instant updates. Set product feeds to refresh several times a day, and switch on automatic item updates in Merchant Center as a backstop.
Finally, decide what happens when stock runs out: hide the product, show it as unavailable or take backorders. Each choice affects search and ads differently, as explained under out-of-stock handling. Accurate stock is also what keeps product ads efficient, which is why my catalogue and dynamic product ads work starts with a feed that reflects real availability.