Online accounts gets locked first the trading and if you continue to play with a hacked savegame then your account will be removed from gamespy, happened to a lot of people already, the thing is people with lots of money start buying cars and sell them again. So cheating ok for offline but for online accounts use at your own risk.
Atari has gone to a lot of trouble to stop people cheating and this is why you cant mod the game which i think is unfair if you are playing offline.
Atari should have Punkbuster and there should be a option in the network configuration for mods offline and disabled online.
This is a very good game and it would even be better if it could be modded.