that's why I mentioned active carts in my post, there are ways to define active cart to get rid of abandoned carts ( ignore carts where last user action was > N seconds ago).

Ok, let's accept the design goal that whoever paid first wins. You can use the same metric (how many milliseconds ago did user click PAY) and impose a global monotonic non-decreasing counter to distribute the scarce inventory. This is how order matching engines work at stock exchanges with HFT orders (FIFO logic).

the goal is to know with 100% certainty, before sending payment request to payment processor, who will have item and who won't, and you dont need to move mountains of rows for that.