Right, but in order to sell high, the market must first be high. He can't afford to wait for the market to rise on its own, so he has to buy to push it up. And in order to buy low, the market must first be low. Again, he can't afford to wait for the market to drop on its own, so he has to sell to push it down. In order for this to work, he has to wait for the walls to be replaced before he can change direction. Anyone who recognizes this strategy can buy into the walls on a change from down to up or sell into the walls on a change from up to down and take the profits the manipulator intended for themselves.