As a practical matter, that is true, and sellers should treat it as such. But legally, I don't think that is accurate. If you tell me you will buy my item for the asking price and I say I accept, that's a contract. Your payment is your performance of your part of the contract, not the act that creates the contract in the first instance. Again, practically speaking, I don't have any realistic recourse if you then back out, but in theory if I really cared about it enough I could hold you to it. And if I as the seller backed out, you could do the same.