If you have the time, another great resource for discovering used books is your local library.

If you become a "friend" of your local library, they'll give you advanced notice of book sales and you get to check out the books before anyone else; that is, you and all the other friends.
My library offers their books, at sales, priced at $10 per bag (and it's a huge bag). Now, I can hear some of you saying that it's old technology and you'd rather use a website but, by the time I finish with my selections, they generally cost me 50 cents apiece or less.
At these sales, I've found really inexpensive books on coins, collecting, stamps, and antiques. Some are used, but most look new (probably just pre-owned). Can't figure out how they get so many new books at those sales, but I'm not complaining.
