I tried several and actually like half.com best. They have a function that lets you put in the price you want to pay for a book, then when the book comes up for that price, it buys it for you automatically. Sellers get an alert when listing a book that they can sell it on the spot for X price, and there are some sellers that would rather just take a little bit less to sell it immediately than wait to sell the book for a possibly higher price. I got most of my books for pretty cheap from there.
