Okay, so in most tales of MC&D Marshall and Carter are portrayed as running the business. So who is Dark and what does he do?
There's some considerable exploration of that over at the Wanderers Library:
http://wanderers-library.wikidot.com/system:page-tags/tag/merchants#pages
It's not tagged as such, all of those articles take their cues from Troy's Groups of Interest page for the Library.
The Library continues to have cordial relations with the most recent incarnation of the Merchants, who call themselves Marshall, Carter, and Dark ltd. They have, in exchange for small sums or copies of lost texts, penetrated into lairs of both the Jailers and the Madmen for us and retrieved necessary items, just as their predecessors. While their two visible partners, Mssrs. Marshall and Carter, believe themselves to be the only such institution, versions of the Merchants have existed throughout time. The Gypsimnum, Shylock's Quarter, and the Black Market have previously served the same role, each of which also believed they were the first. Such is the way of innovation.
The third member of the group, Mister Dark, is a subject of some interest to many members of the Library. Is it known and verified that the same Dark who ran the The Gypsimnum through clandestine donations also maintained several shops in Shylock's Quarter. Furthermore, the method by which Dark communicates is often confounding. Numerous times have books been found in the Library with inscriptions or marginalia signed Dark, often referring to a situation that must be taken advantage of quickly. In each case, we have successfully extracted services from the currently incarnation of the Merchants in exchange for this information, often to the satisfaction of both parties. In spite of this, all are advised to be cautious when attempting to make deals with the Merchants. It's rare that they are satisfied with equivalent exchanges; such bargains make it difficult to turn a profit.
Dark is intended to be the most mysterious member of the trio. He's often thought to be the one behind the true runnings of the group, the one who somehow acquires all of their anomalies, some sort of eldritch being, a young woman, etc.
The tag has a lot of stories involving Dark, including Acquisitions, A Darke Tale, From the desk of B. Dark, Person of Interest - Mr. Dark, and Wills and Ways, to say the least.