The best way to handle book sales for a one-time book signing is simply to offer the books on consignment. You carry the books in, you sell them, and they give you a check at the end of the signing for the actual number of books they collected money for. They pay you the retail price of each book sold less 40% (the standard discount given to bookstores).
If they decide they want to stock some copies of your book for sales afterward, then have them pay for them as well. That keeps the bookkeeping clean and simple.