Step 1, Collect your evidence. This includes the check stub, the ad on RCU, any profile info you can get from RCU, and copies of all emails you have sent and received.
Step 2, Report him to RCU, so he can't cheat anyone else.
Step 3, If the money order was from the Post Office, see if they pursue fraud. If so, it's great, cause that could make it mail fraud, which I believe is a Federal offense. I suspect most other places don't care.
Step 4, Determine where he lives, you have an address to which you sent the money order, right? File a report with his city police or county sherrif, etc.

Note, even if you don't want to name names, you might post his location. Perhaps there's someone else here who can help you with some of the legwork. I'm not suggesting that someone "pay him a visit" (although the thought did cross my mind), but they can look him up in the phone book, give you the contact info for the police dept, county courthouse, etc.