Perhaps I've misunderstood something, but lets take this scenario:
I put up an ad wanting to sell 1BTC for $100 and deposit 1BTC into the "box"
A buyer contacts me, we agree on the terms and the buyer deposits 2BTC into the "box"
I do not send the buyer his 1BTC, but instead tell the buyer that I will destroy his 2BTC unless he sends me another 1BTC directly
The buyer now has two options, either he can destroy my 1BTC and he will lose 2BTC, or he can pay me 1BTC and we take back our BTC from the "box", which means the buyer only loses 1BTC, thinking rationally the buyer will pay the 1BTC and cut his losses.
That is a valid scenario, but one that is unlikely to happen because the buyer can view your historical transactions and see whether or not you've done this kind of thing before, and will take your reputation into account before taking your offer.
You as the scammer, are at risk from the buyer just deciding to just destroy it all so that you lose as well. You do have half as much to lose, but you still lose. You may think it's not rational, but to some buyer, rational option is to screw you over as well if their priority isn't just money.
So you as a scammer have to risk 1 btc every time you want to scam someone this way, and you'll have to create a new account each time to not have any history of bad behavior. Smart buyers will mostly likely not take a big offer from someone with no historical records.
But this kind of scammer is possible, but I doubt there will be such scammers because I don't think it's such a good scam. Fact that I have to build reputation through few transactions, only to be able to do one scam, by an unknown taker who may or may not go along with the scam who may just decide to destroy it.
