You should be able to trade an unlimited number of cars every day. That way I could get all the cars I wanted just by doing one save backup, instead of having to do 4 to 5 a week. I'd also have far fewer PSN accounts.
And to all those keyboard warriors about to flame me, I don't care what you think. GT5 was not meant to be a JRPG, and I refuse to play it like one.
If I want to grind, I'll go play FF13.