This is actually a very good point. I remember in UO vendor malls in the middle of nowhere were quite popular. However, you had means to advertise and teleport (gate) potential customers to the spot. I'm not yet sure how popular obscure/far away vendors will be, but I can see a few POT/PRT gaining fame by having good vendors and people will trek out there for that specific purpose. Still, most vendors will never see a customer, unless they are near a road that is traveled or have an owner who advertises greatly.