There is one left. Two people press Buy within the same second. What should happen?
The comfortable answer is that both checkouts succeed and we sort it out later. It is comfortable because the unhappiness is deferred: both customers get a confirmation, and only one of them gets an apology, tomorrow, by email. We think that is the worse trade. An apology for something you were told you had bought lands very differently from a shelf that was honest with you up front.
How we decide
When you reach checkout, the item is held for you — not sold, held. If someone else gets there first, you find out immediately, on the page, while you still have the option of picking something else. That is the whole point: the disappointment happens at the moment it is cheapest.
The hold is not permanent. Abandoned checkouts release their stock after a short window, which is why an item you saw as unavailable can quietly become available again half an hour later. If that happens to you, it is not a glitch — it is someone else’s abandoned cart expiring.
Why this is harder than it sounds
Two requests arriving at the same moment on two different machines can both read “1 remaining” before either has written anything. Every oversell in the history of online retail is a version of that sentence.
The fix is to make one place, and only one place, responsible for saying yes to a given item — so that the two requests queue up behind each other instead of racing. It costs a few milliseconds. It buys the guarantee that the number you see on the page is a number we are prepared to stand behind.
What we still cannot promise
Stock counts can be wrong for reasons no amount of software fixes: a damaged unit, a miscount in the warehouse, a returned item that never made it back to the shelf. If that happens after you have paid, you get a refund without having to ask twice.
