Are you a car dealer? buy/sell car it's legal, but you must know why the buyer need your service? when people say they will pay for what you bought, the money must be in the accessible account by you, otherwise there is a risk - after you've bought the car, the buyer decide he is no longer able/capable to have that car, you've already paid for!
I'll do it provide the fund is under my control. If you're going to buy/sell a lots of car, I suggest you invest some small money to become a car broker/dealer, that case you won't paying sales tax on buying, but you'll responsible for collecting the sales tax and report the same to your state revenue dept.