Damage caused by intrusion of used cars inspection

Hello everyone. I just had the following problem: I went to a car dealer to look at a used car that is kept in a garage without a license plate. The dealer briefly changed the car in the garage to a place with more light. He switched off the car and put on the 2nd gear, no hand brake! When I wanted to listen to the engine I asked if I could start the car. I opened the door and turned the door shut, standing at the door and it was opened. h what had to happen … the car moved naturally. I tried to catch the handbrake, but knuckled with the left foot very painfully, still hurts very much, and did not reach the handbrake. The car drove about 5m against a wall. Damage: Scratch on the bumper and fender to the right. Who is now carrying the damage? I, who turned the ignition lock (but didn’t know anything about the inserted gear (didn’t even see, since I myself always had the Handbrake used how it should be)) or the seller who put the gear in and not handbrake? And at least not warned me beforehand that the gear was in it! Or partly? Has someone already experienced something similar and knows the right situation? Is the handbrake the legally correct or only procedure to protect against “roll off”? He has now recorded my data and are so far as we are trying to clarify it tomorrow.