Used car sale questions (Frankfurt)

Hi, I would like to sell my car in Frankfurt. Can you please recommend some car dealers I could contact for this purpose in the region of Frankfurt? Please note that I am already aware of the AutoScout24 website:

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Also, can you please help me find a sample contract for a used car's sale? And finally, I would like to know once the car is sold whether I should bring the car plates back to the Zulassungsstelle and deregister the car or whether this automatically done when cancelling the car insurance?

Regards,