A few simple fixes by PD and this can all be gone.
- Lock every car over a certain amount so it can not be sold in game.
- Code all new cars after V1 so it is not recognized in V1 thus can not be sold
DLC can be coded as locked before it is released and all the GTV cars that are gifts can also be locked pre-release moving forward.
As far as catching accounts that have glitched it can be done. I dont know how but FM did it with FM4 based on just scanning game data while you were online. Some how it is all coded. Like I said I do not how it is done but being a long time FM person I have seen this all before. I'm not saying it will happen put there are things that can/could be done to fix this and punish those involved.