Many states make it impractical to buy a car from a private seller due to "safety inspection" laws. In Maryland for example you can't get plates for your car until it has been inspected by an authorized inspection location which just happen to be car repair shops and auto dealers who have an interest in selling you repair services and new cars. Used cars you buy from a dealer come already inspected. It's basically a huge racket to make sure you don't buy a car from any place except a car dealer.