Actually the best thing is spot mining. But it takes a lot of patience. There are times you can get rates when renting from reliable sites better then current payouts.
Right now I am being paid by nicehash at a 0.118 btc a th rate. and at the same time I am renting from westhash at a 0.101 rate.
That is far better then trading.
Hemm... the exchange same with broker only as an intermediary to meet seller and buyer.
yeah, the exchange and a trader get benefit from it.
And any legitimate exchange is interested in their clients earning profits and remaining solvent as long as possible at that (which is in glaring opposition to gambling against the house)...