Well if you speak of price you probably mean something like the actual price at mtgox. Now if you wanted to change the displayed price at mtgox you would have to sell for let's say 50% of the usual price to your own account. But at mtgox or any other exchange you don't trade with a specific account. You always go through the orderbook. So to buy or sell to your own account at the manipulated price you would first have to complete all orders inbetween. At that point it does not realy matter if you buy your own stuff because you actualy did change the price for real.