pretty self-explanatory. But the way you should do it is, buy all the low end cheapest exotics to fill the garage and unlock the achievement, make sure auto-saves are off. Then exit to your xbox dash board then restart. You should have all your money back and the cars should be gone, just make sure auto-saves are NOT on!
Once you return to the game, your reward card for buying all of the cars in this class will be removed also. In order to get the Reward Card Hustler achievement, you will need to gain enough money in order to buy all the vehicles in all 3 classes.