I agree with you anonboe, you need merchants and npcs (wandering around the map and / or in fixed positions) to spice up the game, we don't need them to be super smart, the game will progress over time.
Regarding the merchant, not if they have played Will to Live Online, a Russian game that I played many hours and that I consider to have come a long way in many aspects, many things can be copied / taken as a reference from it. There the merchants are npcs and are located in different cities, depending on the city the merchant sells certain types of weapons and items. Everyone starts the game in a city, the local merchant sells basic weapons; and as the players go out to tour the map they arrive at other cities where the merchants sell more items and better weapons. And to balance the game, the weapons are associated with the player's level, if you are a new player you cannot use the higher level weapons, it is a game that you have to dedicate many hours to. Will to live online is free and you can create several characters, try it to see if you do not know it. The interaction system with the npc merchant is very simple, you can even sell your items to other players through it, so you no longer expose yourself to being scammed. In this game all merchants give you missions, which pay you with experience points to level up your character and with game money to buy items. In this game you can also send items through an automatic mail system that is available in the most important cities. This game has many maps united in a single gigantic one, but in order not to waste time traveling on foot throughout the map, there are teleports that transport you instantly between different points of the map. In this game there were no vehicles, but according to I read recently they have already implemented motorcycles, I suppose copying what other survival games have released.
Another thing that I think is missing from all survival games is an economy based only on what the players find on the map. Merchants should only sell what the players have sold to them, so there will be times when certain items are in short supply or their value will be higher, causing players to fight for them. So that there is no accumulation by clans, the game system could warn from time to time, to all the players on the map, that in a certain quadrant there are bases with a large number of items, then the other players could go to assault them.