The problem with having NPC’s that buy and sell all things, is that would take the economy completely out of the game from a player perspective. if there is no wiggle room for profit, there is no incentive to invest time and effort into it.
That being said (and taking into account that devs have mentioned before their vision is that of a purely player-driven game), I have thought about it a bit and there is a way NPC stores may not screw up the ability to play the economic game.
If devs have stats about what is traded and at what price (stands & baskets only, including hand trades makes it game-able*). NPC’s could conceivably work as long as they are restricted to one half of the market, i.e. either buying OR selling. I think the least impact but most benefit could be gained if it could be set up so that NPC’s would only buy raw materials - things directly harvested or collected. Add to this that they buy at roughly 80-90% of the price these items are bought for in baskets (and/or around 60% of the price on stands) in player-player trades over the last (x time period).
This way, there is always somewhere to sell things, but someone would have had to put in the time to gather these things. It also means that NPC selling would on average be less profitable than selling on the open market, with the exception that they always have some coin. You could farm and mass sell to NPC’s, but as trade in certain items (I’m looking at you flint & foliage…) go down in player-player ineractions, the NPC would steadily drop his price for these until it bottoms at 1.
It should also offer protection agains an artificial price raise in buying resources and waiting for NPC prices to go up, as dumping stock would always mean you only get that 80-90% return (in line with tax).
*If hand trade is included, guilds / cartels / us creative types could mass trade between each other and give back coin after, artificially buffing NPC prices and skipping the deminished return effect above.
All that said, I’m very sceptical that the devs will ever seriously consider NPC’s.