Yoiu learned a life lesson real cheap. Let The Buyer Beware. "Give me your money and wait here; I'll be right back." What if it had been a pound and you had handed him $4,000 for oregano, 'cause you didn't know any better? Well, now you know better.
Whatever you say bro, I'm not gonna argue with you, but when some dude rips you off and you beat his ass, and then he gets ten of his friends to jump your dumb ass it'll be your fault. Violence only leads to more violence, it's not worth it.
Besides we don't need to give the feds more fuel for the fire. A month from now we'll be seeing a dramatized reinactment on t.v. commercials.
Rob his house clean. Rent a Uhaul when him and his family go on vacation, and just act like you're moving out. Anything you don't want, destroy.