Bibliofind
Nine million used and rare books, periodicals and ephemera offered for sale by thousands of booksellers around the world make this the most interesting book-selling site on the Web.

Bibliocity
Rare & Collectible Books from Leading International Antiquarian Booksellers.

Advanced Book Exchange
World's Largest Source of Out of Print Books - 13 million titles listed

Alibris
Alibris is your solution for locating hard-to-find books online. With a worldwide network of dealers and millions of titles, Alibris offers the best selection of used, rare, and hard-to-find titles anywhere.

Seekbooks.com
Seekbooks.com is the first and only Internet search engine for book Web sites.  With Seekbooks.com you can find book related Web sites within specific subject categories, then be linked to Web sites for an in-depth look at books and authors.  You can also buy books directly from publishers on many of the Web sites listed on Seekbooks.com.