We can probably trade with npcs, so I believe it is good to define what can be sold, so that it is not OP.
It will not be possible to buy bullets, magazines or guns, although the player can exchange these items for other things.
It will be possible to buy seeds, fertilizers, construction tools, crafting items such as anvils, presses for making ammunition etc., although these items are the most expensive, all items that will be purchased can also be found on the map without any problem.
Maybe if food seeds and others factors of survarving such as lighter,clothes etc that are needed to calm humans needings in every seasson that treading of such goods that they offer would have any sense to invents your bullets (and other money walutes).