I've had an issue like this when college kid was ordering stuff using the school email/username and ISP, and paying for it with bad checks. it was an Ebay transaction. I didn't know it at first, had the antique radio packed up, emailed him to verify address. He emailed back don't bother I already stopped payment on check hahaha. I emailed back well I didn't ship the radio yet hahaha. I traced the ISP of the email, it was a college, called them, the dean took his side. So then I called campus security, and the local town police where the school is located. Then things changed quick. The dean called me back, apologizing, and they shut off the kid's net service. Turns out he dropped out of college, was still using their free net service to scam on Ebay. he had a long history of buying things and paying with bad checks. so I'd advise you, call the police for that city and tell them what the guy did, because he basically ripped off your stereo. they'll knock on his door and he'll get a nice surprise when they threaten to arrest him unless he returns it. also call his ISP and tell them he's using his net service for theft, they'll shut him off.