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.
But they must predict very well the difficulty increases in next half year, or it will not work. Like if the difficulty stop rising, they will have to pay more Bitcoins than customers sent them