Hang on, that sounds like a serious limitation in the trading platform if you can issue a sell order at one price and have it fulfilled at a different one?

Every trade platform I've used provides two options: sell at current market price, or sell at a specific, unchanging price. So it sounds like he misunderstood the option he was selecting, or something.

That's how every trading platform works. You can put in a limit order where you buy or sell at a certain price, but it's not guaranteed to go through because there has to be someone else willing to take that price. You can also put in a market order, which will always go through, but the price will be whatever is available on the market at the time.

That reads like (s)he was sitting at the computer with the intention to sell at $350, and happened to be there to see it quickly drop to $275, got nervous and cashed out. The unlucky was that (s)he never would have thought to sell it except for seeing the price crash in real time.

One advice: ALWAYS use a Limit order with bitcoins. Both limit buy orders and limit sell orders. That way you're guaranteerd to buy/sell for the specified price instead of the next best offer.