His strategy is to sell high and then buy low.
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.
Joel Katz hit the nail on the head.
With one exception (though he might have implied this when he said 'walls need to be replaced') --
Pirate is the one putting up the walls.
After a rally, Pirate places a buy wall. Why does he do this if he doesn't intend to keep buying? He is simply waiting for additional buy orders to fill in around the support wall. When a sufficient amount of buy orders fill in, then he will sell like mad and crash the price.
After a crash, Pirate places a sell wall, even though he does not intend to keep selling. Why does he do this? He is simply waiting for additional sell orders to fill in around the sell wall. When a sufficient amount of sell orders fill in, Pirate erects a true buy wall as a blockade for additional sellers and then starts buying to fuel a rally.