Until you've actually authorized them to charge your credit card, there is no sale. Requiring vendors to reserve inventory as soon as people put it in their carts would not only be massively complicated, but would also enable customers to indefinitely reserve inventory by simply keeping it in their carts. For numerous reasons, this is not practicable. . . As a point of fact, this isn't much different than it is in brick and mortar stores. Though it would be an absolutely terrible business practice, merchants are technically allowed to not sell you anything they like. By bringing an item to a checkout counter, you are making an offer to buy which the merchant may or may not accept. It would be crazy, but the merchant is legally permitted to decide he doesn't want to sell that item anymore. No one does this, as customer service is generally something businesses like, but if you look, pretty much every store and restaurant as a sign up somewhere saying that they reserve the right to refuse service to anyone for any reason. And they can.