Reclaiming Walther does not endorse all of the material these locations may sell. Both AddALL and BookFinder provide a search engine that is linked to multiple booksellers around the world. While both sites search most of the same listings of booksellers, they are different enough that between them you get a pretty thorough search. Keep in mind that some results will overlap since many booksellers are listed numerous times on multiple bookselling websites. Bookopoly is geared toward individuals who wish to sell books they have around the house that they no longer wish to keep.
Retail booksellers are on the site as well.
The site also contains a Want Ads section where you can list a book you're looking for but haven't been able to find.