Verzeichnis lieferbarer Bücher : VLB
The online database of buchhandel.de is based on the Verzeichnis Lieferbarer Bücher, the most comprehensive database of available titles (including books, journals, and electronic media) in German-speaking countries.

Zentrales Verzeichnis antiquarischer Bücher : ZVAB
The ZVAB is a catalogue of antiquarian books that facilitates the search for books out of print by assembling centrally the offerings of antiquarian bookshops.

The following links do not have a recommendatory character concerning the booksellers, but they can be helpful for finding physics literature.

Amazon.de
Amazon.de provides lists of books sorted by subject, e.g.
Books Physics

Lehmanns Fachbuchhandlung Online
Lehmanns Fachbuchhandlung Online offers lists of current book selections sorted by subjects.