If I'm understanding the question: If you race in a race series you can win every one of the prize cars. You have to run the series four times, and will have to use the "Save/reload trick" to get some of the cars you want.
If you run in a set of races that are simply individual races, you win only one time. If it's a car you cannot buy at the dealer, you might consider keeping it. Cause if you sell it, you won't be seeing it again unless you buy it off of one of your friend's memory cards.