mobile.de is Germany's largest vehicle marketplace, with well over a million cars from dealers and private sellers. It has an English interface (language menu at the top of the page), but many listing details stay in German.
As a foreigner you can search, contact sellers and buy. What you cannot do without German residency is register the car in your own name. CarTurf solves that: our German partner is the Halter, you stay the owner, and registration takes 1–5 business days once the documents are in. Customers contract with CarTurf Global FZCO (Dubai). Our German partner CarTurf UG (haftungsbeschränkt) is the registered Halter.