I've been a member of paperbackswap for many years now and it is basically the only place I get books now (outside of the very rare sale). You post the books that you no longer want and make wish lists of the books you would like. When someone requests one of your books (or however many), you will receive a credit for that book once it shows delivered through USPS. You, in turn, use that credit to order the books you want off your wish list. The only thing you pay for is shipping for the books you mail out.
I've done 21 transactions and all have gone perfectly! I highly recommend this site!