Unlimited arrows, ofcourse. What various people already said. If you run out of arrows, it's going to be a problem. Maybe it's then a better idea, just to sell kind of arrows, but that they can used unlimited. What I mean by that is that people not only can buy a stronger bow, but also arrows with a certain powers, which ofcourse then can be equiped. That way you don't got a problem, that you think that a Fire-Bow is stronger but then go into a Fire-cave with the Firebow. No, you will have a bow with no extra elemental or status effects but you can choose your own arrows. For the Firecave you then can use Ice-arrows.