procedure for selling a car

[reposting with a more appropriate subject]

Hello,

I had an accident recently; the car was completely wrecked (Thankfully, no injuries). I sold what's left of it to a handler. The next day I called my insurance company and asked to stop the insurance. However, they told me that I need to "abmelden" the car; apparently this means turning in license plates. However, the car is not mine anymore, so I don't understand how I can do it. I tried to contact the buyer, but so far without success. Was the procedure of selling the car violated? I just got a piece of paper from the buyer with a signature and a stamp. (When selling it, I actually asked the buyer about stopping the insurance, and he assured me that this piece of paper should be enough, although there may have been some miscommunication - my German is pretty bad...)

Not sure what to do now, or which authority I can contact. Any advice? Thanks in advance!