The OTC is not a market. MMs can do pretty much what they please.
Each trade is a buy and a sell. A "sell" just means the trade was at or closer to the bid than the ask and a "buy" the opposite (it's a question of who moved to meet the other, the buyer or the seller).
The trade may reduce the bid or ask volume. If it goes off at the bid, it usually reduces bid volume, but another bidder may come in, or an MM can buy at the bid without showing his bid volume.
So trying to figure anything out from the bid/ask volume is not only useless, it can lead you dangerously astray.
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