an interesting thought occured whilst reading your post, although the law doesn't force one to sell, if an agreement to sell has been established is there a precedence that states the verbal agreement binds the parties to complete the sale? I know that a verbal agreement is tantamount to sell in the case of housing (in scotland i beleive) but is this an amendment to existing law or is this a new statute?
technically yes. under contract law the contract has been made and if one party frustrates it the other does have redress, however try doing it!
while we are on the subject of legal tender etc, there is no legal obligation for a shop to give you change when you have paid for something