Perhaps this is where to start: BookFinder.com It’s a meta search that searches many of the online vendors. The search results are sorted by price plus shipping which is convenient. If there is ambiguity in the search results, BookFinder presents a list of possible titles.

For example, I searched for Van Gogh and found a book “The Essential Letters.” To verify, I searched the vendors in the list below. Bookfinder’s search results included links to their offerings.

In addition to book finder, here are a few specific used book sellers:
Alibris
AbeBooks
Half price books
Better World Books
Thriftbooks
Biblio.com