why are some cars in Shift undriveable

I just bought a NFS Shift game (for PS3)  from a Play-N-Tra*e (second hand game) store... There are a few things wrong with this game:

some of the Tuning tips contradict each other, and the physics of some of the cars (Manufacturer's races) are ridiculous (Bugatti is virtually undrivable at speed; bounces like crazy) but my biggest complaint is that some cars cannot be used (i.e., in Car Lot, Dodge Viper ), and in certain races some cars won't work (the 'vette in Corvette vs Porsche). I paid $15 for this second hand game, and can't utilize it fully? I'm really glad I didn't buy it new! Very frustrated, and disappointed with EA for releasing a game with such major flaws...