Marker Therapeutics Inc (MRKR)

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I have a question to throw out. A while back it was posted that there were blocks of shares trading at the close of the day, something like 250,000 at a time and It didn’t change the share price each time.
It was said by a poster that large brokerage companies could do this between themselves without changing the market price. When we sell or buy, it drastically changes the price either direction.
Thus My question:
What if the short sellers are large brokerage companies selling short and buy to cover in large blocks from other brokerage companies not changing the market share price? Therefore, they make control the price at the same time making profit.
I’m not a seasoned trader, I just know enough to lose money-lol