As far as I understand they only buy military-related items such as go-juice the simplest strategy I figured out is if you befriended the industrial with drugs art whatever and then just call them in whenever you get a mech raid or anything tough and if you do it right you should get a lot more bodies and money don't worry about the debt getting too high I started paying it back roughly 41,000 principal growth I'm using combat expanded so this might not apply very well to vanilla