I'm going to come right out and say it: Don't do this. I recently wrote about ways engineers can make money, mentioning (among other things) selling epic ammo to hunters. The problem with making the best raiding and PvP ammo in the game only available from players is that it forces hunters to either find an engineer with the recipe who is willing to do all the crafting by mail, or buy it off the auction house.
What do I have against the auction house, you ask? Well, overall it's an excellent tool and far superior to the much nastier alternative of being forced to actually use trade chat for, you know, trade. It's not perfect, however. In fact, there's one massive glaring inadequacy that can be found.