Any road car can be bought second hand in real life. Some are obviously more common than others, but everything is out there at some point, even some a few months old and only 10% cheaper than new. Don't see why a game can't be the same, other than cars never sold, concept cars etc. Classic cars could be placed in a special section specifically for those, it's unrealistic to buy a 1960s Ferrari from a regular used car dealer. With race cars it's trickier, in real life many older race cars are made available and sold, but most are not. You'd have to get the right balance between realism and game friendly.