Great timing on this as I just had to explain this to my wife last week, will link her to this for a slightly more cogent explanation then I was able to give during the movie.

I just watched that movie last night and I really appreciated the explanation.

The Dukes didn't need actually to 'corner' the market.

The number of contracts being sold was exactly the same as the number of contracts being bought. So, "The number of people buying" was essentially the same as the number of people selling.