Paying is irrelevant. Offer and acceptance. Offer to sell; acceptance to pay for the item. A contract is not formed when the buyer pays; just like on eBay, a contract is not formed when the buyer pays. A contract is formed when they press the BIN button or win the auction.
These sells aren't done on ebay.
Most of them are done via email and there is wording about it being subject to allocation. Big box retailers aren't dumb.