> Also: about a zillion successful purchases at yard sales indicate to me that this is a pretty good way to form contracts of sale.
In yard sales (and most retail situations, too), contracts of sale are usually formed with the tender of payment buy the prospective buyer for a specific set of items at a price stated by the seller as the offer, and acceptance of the tendered payment as the acceptance; they sale contract is formed fully executed, not as an executory (with some obligations still unfulfilled) contract.