In the last couple days I have flattened my long and short day trades with Market On Close (MOC) orders on Super Montage. So far I have found it an effective way to flatten positions in small and mid cap Nasdaq stocks. But I am puzzled by exactly how this trading process works. I am putting in MOC orders through Super Montage. My understanding of that process is that my order is matched against other MOC orders that are taking the opposite side of the trade. Since there is only one single price (i.e. the closing price) that will be assessed to both those buying and those selling at MOC, that means there is no spread between the buy and sell side. So how does the market maker (or forum where the trade takes place) make any money on these types of transactions in the absence of a spread?