AbeBooks, (http://www.abebooks.com), the world’s largest online marketplace for books, lists over 110 million new, used, rare, and out of print books from more than 13,500 booksellers.
Amazon (http://www.amazon.com), a dealer of books. Amazon has a very large new, used and rare book selections.
Alibris (http://www.alibris.com), connects people who love books, music, and movies to thousands of independent sellers around the world. Their proprietary technology and advanced logistics allow them to offer over 60 million used, new, and out-of-print books to consumers, libraries, and retailers.
BookFinder (http://www.bookfinder.com) is an one-stop ecommerce search engine that searches over 150 million books for sale—new, used, rare, out-of-print, and textbooks.
ABooksearch (http://www.abooksearch.com), is a searchable book listing database for old, rare, and out of print books.
Barnes&Noble (www.barnesandnoble.com)Taking advantage of vast warehouses across the United States, they stock over 1 million titles for immediate delivery -- that's more titles than any other online bookseller.
ILAB-LILA (http://www.ilab.org), The International League of Antiquarian Booksellers federates 20 National Associations of Antiquarian Booksellers. It speaks for 20 national associations, 30 countries, and nearly 2000 leading booksellers throughout the world.
TomFolio.com, (http://www.tomfolio.com) a website for buying and selling used, rare and collectible books, ephemera, and periodicals
Biblio, (http://www.biblio.com) a global marketplace for buying used, rare, and out-of-print books and textbooks. They have brought together over 5500 professional, independent booksellers from around the world, to offer you over 50 million high quality used books to choose from.