I aggree with the OP, prize cars should not be for sale like in GT3.
It really takes alot of the gran turismo magic away if you can buy every car like in GT5 with enough money.
The X2010 carbon prototype was at start very special and rare, but then the dupers came and raped it.
Just imagine being awarded with a very special car after winning the 24h LeMans which can`t be aquired else. That would be fun!