PaperBackSwap.com is a website for people who want to trade paperback (and other) books. It allows you to post the list of books you have available to send. When somebody sees a book they want, they send you a "credit" to mail it to them. You can then use the credit to order a different book from any other member. (You also get a few free credits when you sign up and post your first nine books.)
I've been using this service for two months and have been quite happy with the process. They make sending books very easy with ready-to-print address labels (that double as book mailers). They will soon be adding the ability to print postage directly on the address labels, which will make sending books even easier.