Keep checking with him... notify the forum admins. Get his IP and see what ISP is leasing it. With that you could get his address & contact the local authorities...

Also ask around on the forum, someone might know something that could help you.

Was it a Paypal transaction? If so you can def get his personal info if you follow the dispute process with them...

Its a pain, but I know I'd go through cause I don't have $100 to just give away.