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.