That requires people foolish enough to pay more for the GH/s than you did. Unfortunately, it seems that there are plenty of those.

Here is the stupid part:

cex.io doesn't even have to mine. Right now, the price of 1 GH/s is about 0.044 BTC, but 1 GH/s will probably mine less than a total of 0.02 BTC due the rising difficulty. When they sell GH/s for 0.044 BTC, all they have to do is reserve 0.02 BTC for mining payments, and they can keep the rest for themselves.

I believe that cex.io has not added short-selling to their platform because they can sell all the GH/s they want for no cost. They don't want short-sellers coming in and taking a big chunk of their profits.