Someone tells me that he wants a particular car.
Knowing someone else who has that car for sale for $500, I tell the first person that I can get him the car for $5000.
The guy hands me $5000, I go to the seller and hand him $500 and bring the car back to the purchaser and say, "Here you go."
While that may be unethical, is it illegal?
What if the guy who wants the car says he'll pay $5000 for it and I take his money and buy one for $4999?
The difference being just the amount of profit?
I genuinely don't know the answer.