As far as Classic cars are concerned, you do find exhibitions that go on especially in the summer. But as far as dealers are concerned, I haven't actually found any in Nantes. Prestige car dealers however are aplenty. Ferrari, Mercedes, Aston, Porsche, BMW, Audi etc.
During the said meetings and exhibitions, lots of cars have "à vendre" (For sale) stickers on them. And a lot of cars change hands around that time. I suppose if you were to buy a car from France and take it to the states it would be rather difficult to have it homologated. European standards for classic cars are rather relaxed. The French like classic cars!:d So they don't make it too difficult to let the car run on common roads