Nate's Food Company (NHMD)

[shotsky]

When you sell at a limit, you get what your limit is, or they don't sell. When you sell at market, you are saying 'I will take whatever you will pay.' So, the mm's sell you a quantity that is shown in their spread, then they immediately drop the bid part of their spread, and maybe up the quantity. And since you are selling at market, they just keep doing that as the price is walked down until your sell is filled.
If anyone doesn't believe that, just sell a million or two at market and watch what happens to the share price.