Buyer complains to me unknown accident damage?!

Hello dear people, I hope you can reliably tell me what comes or can come to me… I sold my car yesterday. So from private to private. As a purchase contract served the form of the ADAC. Just the buyer called me. He would have seen today when cleaning that the car had an accident damage and “was painted. I did not notice this in almost two years. In the purchase contract between me and the dealer at that time it also says: Accident damage – nic I have now given him a telephone to understand that he should have his guesses checked first. In a professional workshop/audit organisation. Law the case that they detect an accident damage, how does it go on? Do I have to accept this and take the car back? Because I don’t really want to… What possibilities does he have and above all, what possibilities do I have? I am in the ADAC and additionally legal protection insured… I hope, jem and can help me…I don’t really feel like such a circus —