I was starting to write a blog post about Paperback Swap --- but a good friend actually summarized it for me a lot better that I could on the ol' facebook:
You should check outwww.paperbackswap.com
(which does exactly what you just described). I have been using it for the last few years as a way to enforce my "one book in means one book must go out" rule. The gist is that you post books you have, someone requests it from you, you pay for the shipping yourself (and can print shipping directly on the site which is something that even Amazon doesn't do), and then you get 1 credit to request a book from someone else. It's pretty amazing."
Being that above is sort of what my piles of books end up looking like -- I've always been a pretty big fan of the site (which really hasn't changed its layout in the past three years... no snazzy upgrades here!"
The nicest feature is the wishlist... You get put on the waiting list for a book that you want, sometimes you can be number 300... but as people start dropping out or swapping that book you get closer and closer to number one.. and then... all of a sudden... BLAM... you're up and you get a surprise book (or really you get a surprise reminder of that book in the form of the ability to request it.. but you know).
While I'm perfectly capable of ordering whatever book I want on Amazon because I am an adult thank you ... this does keep me from book hoarding everytime I hear of a new one that I might like (a concern in my life because I am not very good at being aforementioned adult) . I just add the book to the wishlist. If I'm ever out of books, I could just order that book from the wishlist on Amazon instead of waiting for it on Paperback Swap.... But I've never been out of books.