Used manga sources?

Does anyone have a recommendation for good new and used manga sources online? I'm only interested in Japanese originals, not translations. I found one, http://www.japan-manga.jp/ Japan Manga Bookmart, which does online ordering, sells used for cheap, and ships to America. But they don't have all the titles I'm looking for, and I'd like to do comparison shopping.

http://www.ebookoff.co.jp

There's also http://www.jpqueen.com a nd http://www.jamall.com