I really like the buy-low-sell-high idea, but I agree that an npc should work the people transporting carts. A fee could be paid to him for the pleasure. This also creates another niche in the market, as players could make their own vehicles to compete with npcs; offer faster journeys, cheaper prices or trips to special regions where rare items and monsters could be found (although I think that you should only be able to get so close to one of these areas in a vehicle, to make exploring more challenging).
So everyone\'s happy; people don\'t have to rely on a player always being there because there will be npcs, and Drilixer still gets to be a trucker when he grows up.