And why can he lose twice more coins than the seller? He received his goods already from the seller but he decided to get half of his coins in escrow back. Nothing the seller can do except giving in when the buyer created the deal with a newbie account. He would not care about red trust on that account.
I mean the coins on the MultiSig address, where the buyer locks up twice as much as the seller. In case when the buyer refuses to release the funds, he gets the goods (1x) but loses twice more coins than this goods worth (2x), his profit is x-2x=
-x - not the best situation to lay down the conditions... Yes, for the seller the situation is not better, but at least he will have the chance to punish the scammer and leave him without his money, and I am sure, after that, the buyer will never try to repeat this scenario.