Heresy of Steel Black Market

Me and my group have picked up our Heresy of Steel campaign after a year break and after starting again. No one knows, including myself, why they were going to the black market. Reading the book there is literally no hook for my group to go to it. I went with Thistlemane as a hook and the closest to an incentive is Marcus asking the PCs for safe passage but even then its still vague as heck.

Is there any other mention of the black market during the Belly of the Beast chapter?