Used Books

The Advanced Book Exchange has a fairly large college of out-of-print and hard-to-find books . . .

. . . and Bibliofind has everything else

Also, even though it's not a used bookstore, check out Bookpages: it's like a British Amazon.com. Update: Um, ok. It got bought. Now it is the British Amazon.com.

Other People's Bookstores

FIRE's activism bookstore: featuring all sorts of books about civil rights activism, from all sorts of perspectives

Fun Stuff for Book-Lovers

If you love SF/Fantasy, check out Alexandria Digital Literature: it has a free book-matching program to help you find new authors and novels you'll enjoy, and it also has a very good, if small, collection of published stories for sale. This is one of the better "fiction for sale" sites I've seen—80¢ is a pretty good price for a Nebula-winning novelette

Another interesting pay-to-read story site is Mind's Eye Fiction. The interface isn't as elegant as Alexandria's, but the prices are even cheaper and the selection is impressive. Added bonus: you can read the first part of each story free while you decide whether or not to buy it. You are, therefore, absolutely required to check out "Why I Left Harry's All-Night Hamburgers," the story that got me hooked on SF. I nearly died of bliss when I found it again at Mind's Eye.