The effect of selling is less than you think.
mtgox is not the biggest exchange, but it can easily do a $1M exchange without affecting price in a too dramatic way.
The nice thing is that their bid book is open for every one to see, so you can predict what happens to price at large orders.
At the time of writing, a $1M sell or buy would move the mtgox exchange price 5% in either direction.
If you sell in the largest exchange in the world, which I understand is in China, it would move the market less than this.