The bid will be put automagically by Mt. Gox whenever someone do this kid of trading. The price isn't going to fall below that bid until everything has been bought at that price by someone.
That's not foolproof. If there is a big bubble and it bursts, [...]
If the market is volatile, third parties may spread their BTC purchase offers thinly down to quite low prices... making it more likely there is insufficient depth to cover the trader's loss?
At this point we are beyond my economicsfu, but... if the broker holds some of the trader's USD, is he not better covered for drop in the BTC price? Compared to holding some BTC and trying to sell them in time to avoid loss?
Could someone post a worked example perhaps? Including profit, loss and disastrous loss modes?