Say the long-term, fair market value of bitcoins is around $10, and that's roughly where they would be if nobody manipulated them. To push coins above $10, you have to buy above $10. Anyone who understands your strategy can make you take a loss as you do this -- as big a loss as they want. To push coins below $10, you have to sell them below $10. Anyone who understands your strategy can make you take a loss as you do this -- as big a loss as they want.
Now, say you succeed. You push the price down to $7 and some people panic sell. Now, you have to compete with everyone who knows what you're doing for a share of the underpriced bitcoins. You have no special way to hog them. And say you push the price up to $13 and some people still buy. Now, you have to compete with everyone who knows what you're doing for a share of the better sale opportunity. You have no special advantage over everyone else.
Now, despite all this, you might still think you could make a profit. Maybe, because you know when and how much you're going to manipulate, that gives you an advantage. But the problem is this -- the further the price is from its natural price, the most it costs to move it and the easier it can return to its natural price. So while going from $10 to $11 might not be that expensive, going from $11 to $12 is much, much worse. Not only do you take twice as much loss on every bitcoin. But more people catch on that the price is significantly higher than it should be and more people are induced to sell. And the more easily it returns to its regular price.
I personally know several people who are just waiting for such a manipulation to be attempted and have the funds to profit from it. Any manipulator will have to make them as rich as he makes himself, and he will bear all the costs.
Many have tried this strategy. Billions have been lost.
Joel, you made my day using the Bitcoin Magazine cartoon of you as your avatar. ^5