It's a really cool site and I like the ghash.io pool. Remember its a game of hot potato, as the value of GH will drop to .16 around the next difficulty adjustment. Buy and hold also works, just be careful to calculate your exit point.
Not sure it works exactly like that. I just tried selling my GHS because they were selling for more than what I originally paid. It looks like there is a delay to prevent people from pumping and dumping. Its not instant like when purchasing them. The system seems to just not take back the GHS like you would assume. Maybe an order must be placed at the current price then your sell order gets bundled in.