Every yard that has listed on more than one channel has done this at least once. An alternator sells on eBay at half past two. At twenty to three someone buys the same alternator through the storefront. One of those two people is getting a refund, an apology, and a reason to shop somewhere else.
It is tempting to file that under human error. It is not. It is a predictable consequence of how most inventory tools are built.
The copy problem
Most systems keep the stock number in several places at once: one in your inventory, one in the eBay listing, one in the storefront, sometimes one in a spreadsheet. Those copies are then brought back into agreement on a schedule - every five minutes, every fifteen, every hour.
Between two of those reconciliations, every copy is a guess. If your sync runs every five minutes, you have a five-minute window, all day, every day, in which two people can pay for the same part and both be told it worked.
Shortening the interval does not fix it. It makes the window smaller and the failure rarer, which is worse in one specific way: rare failures are the ones nobody builds a process around.
One writer
The alternative is to refuse to keep copies. Exactly one record owns the count for a part. Every channel reads from it and no channel holds its own number. When a sale lands, the count changes once, in one place, and the channels are told - they are not asked to agree later.
That constraint is unglamorous and it costs something: it means a sale has to be able to reach that record, and it means the ending of listings is an event rather than a batch job. It is also the only version where the window is genuinely zero rather than merely small.
Why used parts make this sharper
If you sell new parts, an oversell is an inconvenience - you order another one. A used part came off one car. There is no second one. The count is one, it goes to zero, and no amount of apologising produces another alternator from a 2019 Transit with 60,000 miles on it.
That is why "never oversell" is in our first sentence rather than in a feature list. For this business it is not an optimisation. It is the whole job.