Indeed, there are several antiquarian bookshops on the Internet, including those in different countries. It is sometimes worth comparing several services that offer the same book.
I myself have bought my older English literature mostly through Abebooks.
Guter Geschmack setzt sich eben durch! Bei Abebooks beziehe ich den Großteil meiner nicht-deutschen gebrauchten Literatur, Booklooker hat hingegen so abstruse Werke wie die Propagandabroschüren des Ausschusses für deutsche Einheit u.a.m.
For all non-German speakers:
Abebooks great for international literature, Booklooker for odd-jobs like cold-war broshures you hardly find anywhere else. Medimops especially useful for used but not so old books, but even though they have international literature too, a German site and I do not know how far they go out of their way sending their products abroad.