Here's how Paperback Swap works. You have a paperback you no longer want. You post it up. When someone wants it, you mail them the book. You receive a credit which you can use to get a book. You pick out the book you want. Someone sends that books to you. That's it.
For more details on the process, check out the Paperback Swap FAQs. By listing 9 books, you get 3 credits to play with immediately. They have a very large selection to choose from, even including materials on cassette and CD.
If you're hesitant about what looks like a hokey pyramid scheme, check out their testimonials, member profiles, and press center (they've been featured on NPR and in Newsday). If Paperback Swap isn't legitimate, someone is going to a lot of trouble for my old Tom Clancy novels. So try it out. By the way, I've been meaning to talk to you about this unclaimed fortune in Nigeria...
Update: Also check out this list at TBR of other book sellers and swappers.