If we allow NPCs to continue selling things but stop them from buying anything the world will run out of tria. Hunting will become useless, since you can't sell the loot. You could hunt tefusangs and riverlings for hides, but seriously who's going to buy hides when they can't make any money selling leather armor? Smithing would grind to a halt, because what's the point of training your smithing skills up when everyone with a decent sum of cash already has 300q weapons and armor? Nobody's going to buy your stuff. If there's nobody hunting, there's no reason for cooks. If there are no smiths, there's no reason to go mining.
If you take out the NPCs, this game turns into a 3d chat room.