Hi there!
Actual bying shouldn't be a prob!
Try
www.mobile.de
www.autoscout24.de
www.bikermarkt.de
If you need more - give me a mail!
The insurance and rego WILL be a prob as others stated before - you better ask the German consulate or embassy in your country how to act correctly - I cannot believe that it is impossible for foreigners to buy and drive a vehicle here!
There are even some U.S. cars with U.S. rego riding around from time to time - maybe you can register it in your home country or something alike!!!
Or you know someone who "lends" his address...
Bye the way - I have a cheap but ugly (technically alright) XTZ 660 for sale at the mo and probably in 2 month a Honda Transalp in good condition (just in parts now ;-) )
So if you're interested - drop me a note - a well - I can help you with German and the other stuff as far as I know either ;-)
Greetings Mel...