Turns out he was a scammer, and Vod was right all along... He still logs into the forum but doesn't post anything.
I don't know if you can consider this a scam...
I mean technically he's just late and assured me he would repay tomorrow. It's not like he stopped answering.
I've put the deal on hold. If he repays tomorrow he gets the collateral back and you can say he's not a scammer. But indeed I have high doubts.