You were already able to buy cars. Go to the docks in san fierro in GTA San Andreas and find the blackboard. You can buy what you bring back to that back.
I know u could do it in GTA III which I also found to be amusing but that had a whole shadiness factor to it, afterall u weren't buying a car from a dealer or anything like that. I would like a decent empire building system similar to what scarface the world is yours had. It seemed no matter how many Kgs of coke you sold you always wanted more $$