Most libraries have an area where they have books that they're selling. I don't know is this is cheaper than going to yard sales and such but it's still pretty cheap! And you can gets books, like science fiction books (if you're into that) that people don't usually sell at yard sales. This is good for avoiding shipping prices.
Ebay is also a great place because people sell any books there all the time. And there's always more auctions then people bid on. I've bought lots of 20 books for .99 before. Watch out for shipping prices though!
Dirt Cheap Books for the avid reader
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