This is not always the case anymore with MM's

years ago teh money was in the spread so you always bought from an MM at the ask and sold to an MM at the bid so stock always changed hands thru 2 MM's and OTC volume was double counted.

Today this is not entirely true. Many firms have MM's on sire for order flow and make money on sec lending and margin rather than on the miniscyle spread. Many firms therefore match trades in house ( suppose Etrade has 2 client selling and anotehr buying at market or same limit) they will match the trade in house and not go thru 2 MM's. Likewise when I use amreritrae and go inbeteween the Bid/Ask I see my bid up there and then when I buy the bid goes away. So only 1 mm is involved and the shares are not always double counted. Likewise for ECN involvement