I suppose the real answer to the OP, and I apologise if I'm just simply recycling previous posts, is you need to ask the question: "How much am I willing to buy/sell BitCoins for?"
That is your personal exchange rate, if somebody else agrees with you, then you have a sale.
As said before, it doesn't matter if all the offers there are artificially inserted by the exchange themselves. The only problem comes is if they are selling BitCoins they don't have, in which case the problem is something else entirely, and not manipulation of the market.
I would phrase it the opposite way. If someone disagrees with you then you have a sale. If I value a coin more than 40 cents and so does someone else we have no trade. But if they value the coin less than 40 cents we can both benefit from the trade.