I hope this works out for you. I got scammed on Ebay once for $2200 and it took almost a year to get my money back.
Not sure why you are expecting to get more from this guy than what you paid him. You'll be lucky to get your original $$ back. A four week delay in shipping doesn't put some sort of penalty payment in play here. He owes you an exhaust system or your money back and that's really it.