I'm an advocate of buying books new whenever possible. Writers don't make royalties off second-hand book sales after all.
That's not to say I don't frequent used book shops because I love them and I love finding forgotten treasures in hidden corners. And writers know they increase their readership in used bookstores, which ain't a bad thing.
The web of course opens up whole new avenues for finding lost treasures. Ebay has made just about any book printed since the beginning of time available to bidders. I've added to my Doc Savage books via ebay and online used bookstores.
Online book trading came to my attention via Library Thing. They link to a number of book swap sites and people talk about them on the Library Thing message boards.
I'm test driving Book Mooch because it seems user friendly, becuase I'm a biblioholic and I have some books I've read that I don't need to keep inventoried, and I also have duplicate copies of some titles.
The most important of those reasons is: I'm a biblioholic.
Apparently you list books you're willing to trade. Other users request books from the list and you mail them out book rate.
When you send a book you earn the right to request a book, and someone mails one to you.
My first request is in - so I'll be dropping by a mailbox today.
We'll see how it goes.