Sell logged in to export dealers?

I would probably sell my car to an export dealer, I don’t think I get more than 500€ anymore, since I currently have two cars, I won’t give it in payment for another one like the last time. Can I trust with an export dealer that he also quickly unsubscribes the car (he advertises regularly in the newspaper and that for years). I didn’t have space to leave the car standing unregistered. I read that I have the car with a copy of the purchase I would give the dealer the car and tell him that I will have the car insured one more day, I would inform the insurance company in advance. In addition to the purchase contract, I would write a letter that the dealer undertakes to unsubscribe the car promptly. Opinions/recommendations?