I'm pretty sure that you have to have an address in Germany to register the car there. I had to provide proof of residency and my insurance details when collecting my number plate.
Unless its something very special (i.e. rare and very expensive) it would just be far too much hassle and given my experience of German bereaucracy I suspect it isn't even possible without being resident.
Yes, you need to have a residence in Germany or someone who has. The Austrian market is much smaller than the German (and it's much further away from GB). If it is really profitabelk to sell the car in Germany, I would suggest to ask someone who's living here.
