WooCommerce eBay integration that keeps one stock count

Items pair up by SKU, so the shorter of the two limits decides what your naming scheme can be. WooCommerce will store a SKU up to 300 characters, which is more room than any catalogue needs. eBay takes 50. Anything longer is truncated on the way to eBay, and a truncated SKU no longer matches the item it came from, so the count for that item stops moving.

Nothing errors when this happens, which is why it catches people out. We send the shortened SKU rather than refusing the item, so no listing turns red and no warning appears anywhere. The item simply stops pairing with itself, and you find out weeks later when the eBay count and the WooCommerce count disagree by a few units and nobody can say when they parted.

Fifty characters is enough for a sensible code and not enough for a sentence. If your WooCommerce SKUs were generated from product titles, that is where the trouble will be, because nobody ever had a reason to shorten them. Export your catalogue, order it by SKU length, and deal with the top of the list in one pass before you connect anything.

Some channels announce a sale the moment it happens. eBay is not one of them: it has no way of telling us anything, for stock or for orders, so we check it on a timer every couple of minutes and pick up whatever changed since the last look. WooCommerce is the opposite and announces its own changes straight away, so this pair is live in one direction and checked in the other.

In practice the window is small and it is not zero. If the same item sells on eBay and on your WooCommerce site inside the same minute, the eBay sale is waiting to be noticed while the website still believes the stock is on the shelf. A unit or two of safety stock on your fastest movers covers that gap, and planning around it beats assuming it away.