A few weeks ago I heard about this service. It's all on the honor system. You post all your old paperbacks at this online site and you can trade them straight across with other folks who have paperbacks to swap. I've been using it for the last few weeks and so far it has worked like a charm! I cant recommend it enough. it kind of works like this:
I have an extra copy of Conan the Conquerer.
I post it on the paperbackswap database.
someone, somewhere who is looking for a copy of Conan the Conquerer sends me a request for it through the paperback swap database.
I send him the book (I pay Postage).
I then get a "Credit" to use when I see a book on thier database that I want.
I see that someone has "King Kull" up on the database. I use my credit to order it. it is sent to me (sender always pays postage).
Give PaperBackSwap a try. and if you do try it tell them "Boojiesdad" is your referrer, I get credits for spreading the word.