I think it would be a good idea if after interrogating a faction leader, or studying one you'd get an option to buy items from said faction for a high price.
Like 50K guerrila gear from the Spartan Lieutenant, 20/30K Rifles from the Spartans, 100K Laser guns from the church. Etc.
While some stuff can be made like integrated devices having the church sell them each for like 10K to you before the research option appears, or durathread mesh makes some sense.
Other items like the courier newspaper should be on the market as well since the white normal market undoubtedly sells it too.
While the main faction might not approve of such sales, individuals often times are motivated by greed if anything you could simply contract the mercenaries to buy the stuff for you to explain away why someone is selling you the items.