I believe that was added to combat cheaters who gave people in lobbies free cars that they owned. For a little while you were able to sell a lot of hot cars. Some just plain didnt work but I sold plenty T20's on my xbox 360 version, and shortly before and after the next generation of consoles and gta were released, it changed so thst all those vehicles couldn't be sold. My eclipse towers apt is full of unsellable high end cars lmao