Dealers selling privately

Hello, I have a few acquaintances who have been trading with cars for some time and therefore also earn a lot of money. The best shops do those with the purchase of cars with engine damage, which are then repaired under the hand and then sold privately (without warranty etc.) further, so that margins between 3000 and 5000 Euro are in it, which then are not even taxed. How high is the chance that one is caught thereby? I see little risk but the possibility of money to earn… In general, this is not standard for commercial car dealers? Selling a few cars commercially and otherwise private? Greetings Tom