Can you show the link to your auction?
Also, as this post said:
You can, though I'm unsure if it would be properly classed as a scam as no money or items has been changed hands.
If he isn't responding to your PMs it would perhaps be a way to get his attention, but I wouldn't expect too much more to come of it.
So I think you should re-open your auction, or ask the 2nd highest bidder to buy the item.
In my opinion, you should ask low ranked members/negative trust members sign a message of address(es) holding at least the amount he bids. This can prevent that kind of bidding.