This is the reason we should all want to invest in, or attempt to design, an open source completely information-transparent exchange. If a buyer has large enough funds and buys/sells enough to push the price above or below the spread cost to net a profit, then he/she will do so. This is a runaway scenario because they profit at the expense of others everytime (no negative feedback) to greater excess than the average trader. They could potentially propagate this strategy essentially ad-infinitum.
The other scenario is the "exchange" trades his/her own account and spoofs trades or doesn't pay the spread or has relatively larger working capital than the aggregate buy/sell trades to push the price in the direction desired, in addition to avoiding the spread or other associated fees. The only other option is to be the bigger player in the market, but that technique is still flawed.
Openness is key. That's why the bitcoin protocol works in the first place (the blockchain is public and so are all the defining algorithms and source code). Any degree of clandestine trading of bitcoin (centralized authority) for other fiat currencies (or any commodity really) that doesn't play by the same rules can be easily gamed given enough relative capital.
If the market weren't so small...
EDIT: Of course, it's hard to tell if there is one participant pretending to act like a "multi-participant" thus acting "in concert" to influence/manipulate price direction due to the psuedo-anonymous nature of bitcoin. What do you do?
