Car deregistered in 2008 and today 400€ bill of insurance

Hello dear forum, I made a typical mistake with my first private car sale a while ago and sold my vehicle unregistered in June 2008! In the purchase contract we had agreed that the buyer cancelled it within 4 days. Of course he did not do that and when I came back from vacation it was still not logged off in September and I am with the purchase contract to the registration office and these meant to unsubscribe. As a result, I also sent it to the tax office. I’ve received the remaining taxes back and also canceled the insurance by phone. So now see there in my account missing 400 euros which the insurance has deducted me. On the phone they told me that they received a notice from the registration office of the buyer with cancellation date March 2009. The registration office of my city said that this was probably because the city of the buyer had only to find this license plate (6 months apparently) etc. . Ic h doesn’t want to hear how stupid it was of me… but I’ve had a lot of fun all in all that cost me over 400 euros. I have a purchase contract (he didn’t want none for himself). What am I supposed to do, do you think I have a chance with a legal lawsuit? The guy seems to have money already. Should I file an ad with the police or a lawyer? I’m already in despair, this sale is following me now just over a year!!! Would it bring something w can I present the purchase contract to the insurance company? Or am I simply still their contact person? Would be really grateful on advice!!