I think there should be cheap mid-tier ammo available from NPCs, and then let crafters make the very best stuff. Let rangers and corsair just buy cheap and "all right" ammo for use in exp parties and lower difficulty NMs, and then they (or we, when i eventually get rng99) could spend extra money on the best ammo when it really mattered, such as on tough NMs and events where only maximum efficiency is good enough.

NPC ammo could for the most part have no stat boosts, just a dmg rating, and that dmg rating could be slightly lower than the "equivalent" crafted ammo. Additionally, you could add NPC ammo that had the same DMG as the best crafted ammo, but had stat penalties that made them unfeasible for use in the real hard endgame activities. For example, a lv99 rng with capped skill and accuracy bonus traits isn't going to get below 95% hit rate on DC enemies even if they equip ammo that has -10-20 ranged accuracy, but you would probably not want to use this kind of ammo in the events that actually matter. In these events, we would pull out our high-end, 100k-a-stack ammo that we could only get from crafters.