this kind of merchant only buy and sell some types of items.
for example: a trapper NPC in klamath who sell and buy pelts. the barter system will be made through dialog options.
first select buy o sell option. from sell option you have to select the kind of pelt, an then an option to sell one, five or all you have in you backpack.
from buy you will receive an answer like that: "i have 53 regular gecko pelts, $50 each and 18 golden gecko pelts, $150 each." (prices are modified by barter)
you select golden or regular option and the amount (an option for buy one, five, or ten)
When you sell or buy from that NPC, his stock will be modified.
There will be more pelts merchants in other towns like NCR, Broken Hills, Modoc, etc. but in klamath geckos are more common, so the merchant in klamath will be the best prices for selling and buying (0% in barter skill), the NCR merchant will be higher prices (50% barter), modoc 25%, and Broken Hills 70%. or some like that.
So players who wan to buy the cheapest gecko pelts will have to go to klamath. same for selling.
This will be done for all the raw materials, and some components, like alloys, gunpowder, metal parts.
Merchants special quest:
This NPCs will have a special quest to merchant players (higc barter and CH) when they have a large amount of items in stock.
for example the trapper NPC in klamath, will ask some merchant to sell his pelts to another of the skins merchants. The player have to buy a X amount of skins and bring it to the other merchant .The other merchant will pay more (barter) for those pelts.
May be a requirement will be have a caravan cart, and the pelts (or whatever you are buying) will appear in you cart, and you have to bring the cart to the other town, and speak with the other merchant to complete the quest. (yes, you can die travelling and losing all, but that’s the life of a merchant in the wastelands).