Used Book Central 
Searches the stock of affiliated dealers in the US.
http://www.usedbookcentral.com/Bibliophile
Multilingual book search engine.
http://www.bibliophile.net/
TomFolio.com
Searchable database of used, rare and collectible books from a group of booksellers.
http://www.tomfolio.com/
Bibliopoly
Multilingual, international search for antiquarian and rare books.
http://www.bibliopoly.com/
Abebooks.com
Offering new, used, rare and out of print books, textbooks, and ephemera. Represents a wide variety of independent booksellers located throughout the world.
http://www.abebooks.com/
BookSearcher.org
Large online database of used, rare, hard to find, out of print, antiquarian and collectible books.
http://www.booksearcher.org/
ABooksearch.com
Searches databases of participating booksellers.
http://www.abooksearch.com/
Big Words
Compares textbook prices at four stores - buying or selling.
http://www.bigwords.com
Cover to Cover Antiquarian Books
Offering a range of antiquarian, second hand and out of print books.
http://www.covers.freeuk.com/
Antiquarian Books in Scandinavia
Search the databases of the Scandinavian members of ILAB, International League of Antiquarian Booksellers.
http://www.antikvariat.net/
Used Books Search
Search for used, rare and out of print books.
http://www.usedbooksearch.co.uk
Ardis Online Database
Searchable database of secondhand books from UK booksellers.
http://www.ardis.co.uk/index.html
Biblio.com, Inc.
Searches databases of associated independent booksellers worldwide.
http://www.biblio.com/
Library Book Sales
Matching book buyers with quality books that have been donated to libraries or are duplicate copies, monographs or surplus materials. Ex-libris titles are discouraged but not excluded. Library types participating: public, private, institutional, special collections, educational, foreign or domestic.
http://www.librarybooksales.org/
Biblion Ltd.
Antiquarian and rare books from primarily UK book dealers available on a searchable database.
http://biblion.co.uk/
ZVAB
Search for old, rare and out-of-print books. Access to many bookstores. Bilingual, English and German.
http://www.zvab.com/
