Do you have a picture or copy of the money order? Or receipt from the USPS that you sent it to the seller? Emails, phone records, etc.?
Send him a certified letter with signature requirement (so you know he received it) in the mail requesting that he either send you the item or fully refund you within 10 business days. Include all your emails and evidence. You can choose to state in this letter or the next one that if you don't receive either you will take him to court. Maybe that will rattle him. If not, take him to court. Depending on the jurisdiction you may have to serve him in his home county but this is over mail so you'd have to look into that. You'd also have to see if its worth taking to court depending on the dollar amount.
Good luck in your efforts. It's a shame when people do this type of stuff.