Have you ever been in a shop during the new-year sales? There can be a lot of demand for a limited number of physical products but there can never be more items on the shelves and in shopping baskets than the actual physical number of items there were for sale in the first place as the atoms of the items themselves prevent duplication.
It's not quite the same for virtual shops; there are no physical products in the webserver - just bits representing them. By careful programming we have to provide safeguards against race conditions ourselves. Threadsafe counters provides one method for doing this based on an escrow model in which sales are not a 2-state affair (available or sold)