Buyer does not declare car as agreed

Hello, I recently sold a vehicle. Purchase agreement is available. I photographed the buyer’s identity card when it was on test drive. This looked serious. Buyer would have to unsubscribe the car within 3 days according to contract. Now this does not report any more, questioning if I can reach it again. Insurance and registration office I already informed about the sale on the day of the sale. The insurance terminated the contract the next day. i can’t reach the buyer anymore. How do I proceed? Criminal notice? ID card is available (if it’s real at all…) What if the buyer is building an accident? The insurance is terminated. So far I have always sold off – probably should have kept it.