If you didnt sign anything he's not legally obligated to do shit. What is it exactly that you did for him that is worth $500?
If it were me I wouldnt be asking for 500 from the buyer. I would be asking for 500 from the seller. If I sold your car for you, you could give me a cut. But if I had a friend with a car and I got it sold for you, I wouldnt ask the buyer for 500 because I got you the car.