Or maybe limiting the merchant to only buy specific item. E.g. only cookedalienmeat and cookedfish. I'm currently trying to make an NPC that only wants to buy specific item/specific type of item from player. For example, a butcher would only buy raw meats from player. Apparently bmain.lua and the base merchant.npctype don't have anything of that sort... Any clue where I should look at?
Well it wouldn't be in bmain.lua etc because what makes a merchant is their interface itself - which you can think of as being separate from the NPC. Scripted interfaces have their own scripts/script environment. I think you'll find a satisfactory answer in /interface/scripted/cropshipper/