Depends on how hard/long the race was, and if I can or can't win it again by doing a different race.
If it was a very long race, and a pain in my a__, AND I can't win that car by doing a different race that's easier, then chances are I'll keep the car.
If the race I won the car in was short and easy, then I'd sell it if I were you.
P.S.-I wouldn't sell any car I have, unless I already have one of them, or I really need money.