A user who submits an offer should have to put their payment and shipping info in when submitting an offer so that the sale will immediately go through if it is accepted. Otherwise, the offer system is prone to abuse by buyers with no real interest in actually buying. On the flip side, it also allows a book to be bought by someone else while a bundle offer is pending, meaning I lose out on selling a bundle of books and the potential buyer losing out on a great deal. Then there's the issue of multiple people making offers that include the same book.
As a seller, when I send an offer, I've committed to the sale. Buyer-initiated offers should be treated the same.