So I haven't played in a couple of years

and naturally a lot of things about this game have changed.
I'm starting to figure things out on my own which is fun, albeit slow.
However, one thing that is new that I can't seem to figure out: Merchants.
Specifically: What do they buy? Is there a guide to what they will buy from you?
I've perused the wiki & searched the forums a bit but can't seem to find any clear answer.
For example some merchants will buy my weak healing powders & brahmin hides & some will only buy guns/ammo.
Some won't buy anything at all and I've yet to find any merchants that will buy armor from me.
Thanks for your help.
(Also, if you can think of any BIG changes since 2010 that you would like to mention, please do!)

PS I have charisma 3.