PaperBack Swap allows you to post your books and trade other users for their books. How it works is you post your books and when someone wants it – you send it to them (pay the 2 dollars in postage) and they get your book. Once this transaction occurs you get a credit which allows you to request a book from a user and they in turn send it to you. I think when you first start you get one credit for free but I could be wrong.
I have used this service quite a few times and it is awesome for those who enjoy physical copies of books and don’t mind waiting a few days to receive them. Plus it’s basically free (minus postage).
Check ’em out.