Accident damage to the previous owner

I sold my SLK 200 Bauj. 98 1 year ago as accident-free (as in 2007 with 135,000 km from 1st hand bought) with 160 000 km and was addressed by the buyer directly by lawyer that the vehicle had an accident damage. A simple report was attached The damage was determined because of an engine defect. I drove the vehicle to the TÜV myself 1 week before and no defects were fixed. Since there was no mess, I have a more concrete report e It was found that there were several accidents that were probably related to datam 2002/2003. Jezt should still be named the previous owner as a witness, who of course will not admit the accident. he already does not appear 2 times. How to deal with it now.Kamm one by a paint analysis exactly find that the Unall 2002 was and in the time of my possession (2007-2009 ) could not have been at all. I had no accident but the whole theater makes me still curate. If I still have a car and buy case-free and had no accident, I sell it cook again accident-free. I used an ADAC purchase contract. Would be glad if I got information. Greetings Dagmar