You can go to Service des Automobiles in vernier with your Carte Gris and cancel it, then you can get your plates and throw them in the plate bin. You get back your carte Gris with a stamp "Annulee" on it. You'll inform your insurance, then will be refunded the amouth of insurance (if your insurance is from 1 Jan-31 dec, you'll be refunded for the remaining days). Same with plates. Service des Auto will refund you for the remaining days for the tax you pay for plates.

New owner can go with your carte gris and switch it to his name. As its an old car, for him to register, he'll need to take it to visit technique, but it's his problem.

The money transaction: you will do between yourselves, I don't know what is the best most secure legal way to do it. In all my transactions I just paid owners and they gave me old carte gris.

Thanks. Can I cancel the carte gris *after* selling?

It's just that he wants it tomorrow and I need to wait until at least Monday to visit the service des auto.

 I am pretty sure the new owner does not want to re-register it but export the car. ITC he shouldn't be needing my carte gris.

I guess you can, as selling is just money transfer between you and him. But it seems slightly suspicious to me that he needs it that quick and did not even see the car... And before you cancel it, I think it is still officially your car.

Hmm...I didn't think about it much. Am eager to get rid of it early too, to save the pain of having to pay the tax+insurance for next year.