I won't keep cars just because I win them. I collect cars that are of interest to me (Still looking for the right 2002tii), but I sell cars that have no intrinsic value (Toyota Yaris RS).
I'm somewhat collecting, but only for the purpose of the 1000 car trophy and other unlockables received for possessing a certain amount of cars. Therefore if I do need cash I am always happy to let a few go for a quick boost to the bank.
I sell the standards and keep the premiums.
keep everything, gonna need every car I can get for the 1000 car trophy.