You make a market in a stock--which I have done for a living.
Merill comes in for ten thousand. You are flat. You sell him whatever. two thousand, three thousand, five thousand; maybe all ten thousand
You go to your sellers and tell them you are a buyer. They tell their salesman who has been a seller and maybe/maybe not he comes up with stock and you make a profit, having sold on the offering and bought back on the bid.
But if you sold Merill stock and weren't long to begin with, and couldn't find a seller, you are SHORT, whatever.
The stock keeps going up. You keep getting shorter because you have to appease your customer.
When it starts to come in, the stock you buy, you sell in the market to 'make it come in some more'.
If you are real short--and keep in mind there maybe multiple market makers in the same position--everybody keeps selling till it could go down below where it started.
That's how they make a living.
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