Suppose you just bought a car off of some person on Xbox Live and you don't have it unlocked. To copy a car, you must have at least 2 profiles. Let's say the profile with the car you wish to copy is named profile A. Your secondary is named Profile B. Load up Profile B, go to Garage and select Profile to Profile. Then click on the name of the profile that owns the car you wish to copy. Find the car, and buy it from the profile, it will stay in the profile you bought it from and you will also have it in your secondary profile. Now that you have done that, you can load up Profile A and buy that car. Now you will have 2 of that car, and you can continue buying it.