Quick version (heading out to dinner): For every buy, there is a sell, and for every sell there is a buy. It depends on the buying pressure, or selling pressure, and the way the MM match up the buys with the sells to make the deal, and where the bid, and ask are at, whether is shows in green or red (buy or sell).
It is a lot fuzzier in the pinks than the big boards. In the pinks most buy at the bid, and try to sell at the ask. On the big boards I do not recall ever being able to buy at the bid, and sell at the ask, unless they are at the same price.
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