Look, this whole argument is actually in your favor, man. You have proof on a public forum that this little idiot who initially posted derogatory statements about you on this site has lost you business. That's libel. And you have a hell of a lot of evidence right here. Forget what he owed you for the car, wait til you see what he owes you when there's a judgment entered against him in court!! Contact me, I'll give you the info.
I've owned a business for 12 years and I've dealt with all kinds of idiots like this kid. Professionalism is fine, but there comes a point where you have to fight fire with fire. We'll talk.