If all types of merchants have a different item that can be sold to eachother using a scissors beats paper, paper beats rock, rock beats scissors kind of loop then it would make items more necessary to keep and sell regardless of your character.
For example, you have 30 stealth boys, you can sell these to a medical merchant for placeholder which can be sold to ammo merchant for placeholder2 which can be sold to armor merchant for placeholder3 then to weapon merchant for placeholder4 which can lastly be sold back to medical merchant.
It's a good idea but is it too trivial?