sorry to hear that
, If he was on the forum, he was connected to a computer to do it, which means the site should have a log of his IP address. if the guys was smart he would have done it at a library but chances are he's not and he used his home computer or a friends. Track down the IP address and see who its registered to, you may have to go through internet providers to find a billing address. Don't lose hope, there is alot you can do to catch this guy if you go through the proper channels.
My suggestion is make a new name on the forum and make a fake offer of having a nice laptop to sell/trade and go through with the deal only instead of sending the fake laptop send a tracking device, almost anything can work for this, a cellphone, a garmin gps even a jerry-rigged lojack device. send it to his house and you got him. good luck keep us posted