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For every trade there’s one buyer and one seller

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Not exactly. The market maker is the middle man on every transaction. If you are buying shares, you are buying from the market maker, when you sell shares, you are selling to the market maker. That's why you often see what is termed "mirrored trades". It's the market maker buying and the market maker selling the same group of shares.

Sometimes, often more than 30% of the time, it's the market maker selling and the market maker buying the same group of shares.