"if someone is selling, someone is buying."
That isn't exactly true.
By law, MM's have to buy shares if they are offered within their spread. That doesn't mean there is a buy order in.
As well, MM's must sell if a buy order comes in that meets their spread and they have the shares to fill it. It is in this case that some mm's short the stock, by filling the order with borrowed shares. They try to cover in the same day, but if they don't it gets put on a short report to finra, twice a month.
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