Hi
Actually you can now do it through your mairie and the easiest is that you ask them advise there if you're not in a big city.
You need to give them the old carte grise (or certificat d'immatriculation as the admin people will call it) cross with the note "VENDU LE (DATE) and "TIME) and you also need to provide the copy of the CERFA that say "Exemplaire prefecture" and a valid less than 6 months old controle technique.
You should also ask the seller for a certificat de non gage(meaning he doesn't own any money on his car for a credit or something else). Ask Bev said there are 3 copy of the Cerfa (one for the seller to keep, one for you and one for you to hand out with you paperwork to the prefecture).
Happy buying.
Cheers,
Pauline