This is not a "beartroll FUD, OMG U NEeD CHEAP MY CHEAP COINS" post, its a simple argument:
A) Imagine that you are running an exchange that is so big, that every other exchange is simply following what you do. The price is pretty much attached to your action.
I think this is a reasonable claim. If you monitor the current price action, the western exchanges never lead, it was not like this in the past, there were actually serious actors, and were leading the price with china following them, and vice versa. But not anymore, now the scene is dominated by OKcoin/Huobi.
B) If A) is true, what should happen naturally is this :
A big miner, or most likely a group of miners will approach you with the deal.
What is the end game for the miners? As any investor, they want max ROI % on the investment. What is the best way to do it? To make the BTC price high and sustained.
If someone has the potential ability to control the price (OKcoin/Huobi), miners absolutely MUST try to bribe them into the manipulation(if they are know what they are doing as the investors). Its a very simple game theory move.
Now, any reason to OKcoin/Huobi to deny the deal?
Long term price manipulation is basically impossible in any market.
Remember the good ol' Mt. Gox days? That was a good example
of when there was one centralized place that published the price.
However, the situation is far more decentralized today. If any
exchange tried to manipulate the price, they would have to
continue to pay for it because arbitragers would take advantage
of the situation.
If the price was too high, I would buy lower elsewhere, and
sell on the manipulating exchange, and if the price was too
low, I would buy coins on the manipulating exchange and
sell them elsewhere.
The only reason people couldn't do this with Mt. Gox is
because they couldn't get their money out! lol.
If there really was only a single exchange, it would be
easier to manipulate the price in the short term, but
would still be playing a risky game, either buying higher
or selling lower than market prices.