Major savings are to be had and great for hard-to-find titles. Beware the hidden cost of shipping: be sure to compare TOTAL price, as some dealers offer very cheap book price and hefty shipping charges. Others include shipping in the book price. Also, try for dealers OUTSIDE New York State, to avoid state sales tax. All in all, a very thrifty way to stock up on books.
ACSES This site will compare prices from many sources, including postage, to show cheapest total for one book or for a total order of several books.
ABEBOOKS Advanced Book Exchange
BEST BIG ONLINE BOOKSTORES
ARACADABRA BOOK SEARCH INTERNATIONAL
ABAA Antiquarian Booksellers'
Association of America
International League of Antiquarian Booksellers