There is a misconception that MM's have to actually own stock to buy or sell. They are allowed to make their markets at any time and then settle up with others at the end of the day. This is done to make sure there is liquidity in the markets. It is up to the individual MM to police the trading. That is what is meant when the MM is trading for their own account. For instance, they will sell 100 shares in the morning, then buy 100 shares later that same day.
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