AddAll - a consortium of used book sellers
Advanced Book Exchange - for out of print books, links to many bookstores throughout the U.S.
BooksPrice - finds the best price on a purchase of several books together
Chegg - a used book resource
FetchBooks - a consortium of sellers dealing only in books
John Gach Books - a bookstore in Maryland. Stocks many out of print classical psychoanalytic works (somewhat pricey)
New Books in Psychoanalysis an online program devoted to interviewing psychoanalysts about their latest publications.