From what I've read the Tank was purchased in England and imported to Germany.
I read the German newspapers about this, and did not get that story. However, the 78 year old guy has a permit dated 2005 for it, it has been demilled (cannot shoot) but the police is not sure if that permit is still all valid today. They are investigating if he is violating the "War Weapon Law".
It is a bit like Gibbs in NCIS; how the heck did he get those things into that cellar?
OK, quick update - editing.
The cellar is an under-ground garage located under the house. Problem seemed to be that the tank had to make several sharp corners to get out.
Under this link are some more pictures. And the guy definitely was violating the lwa with those things, even when demilled.