This has been discussed a few times under the black market topic and of course opinions vary. The open black market as we see it today is largely just cash sales by entrepreneurial types, not criminals. I agree with you that essential resources will be controled by hardasses looking to get rich off of the suffering of others. Most of these people will extort as much from you as they can. So are you willing to give up everything else to get that item? If you have never dealt with this element of society in the past it’s probably not something I would be considering now as a posible venue of supply. What ever you think they will have that you will need, get it now.
If you must have interaction with criminals you can expect to get ripped off and placed in danger. There are true psychopaths floating around willing to hurt you for fun. Lots more than you think. These guys are why you need arms to start with. If you are somewhere low life’s frequent you never know who will walk in the door. Most low life’s are willing to rip you off and hurt you, but there are other guys that scare the sh-t out of the average dirt ball that you don’t want to be anywhere near by when they show up.
One of the edicts of the drug trade is to never sell to someone you don’t know, that way you can’t get popped. I would never buy from someone I didn’t know in a SHTF for the same reason.
That’s my 2c