Maybe say what they are. If they have limited availability then they are meant to be rare and a bargain. You will just have to camp the vendor orgo at a time when other people arent there if you wnat the bargain price. Without you saying what the item is and preferabley which server its hard to tell whether you are complaining or just lazy.
most of the limited mats i've come across at npcs seem to be very low level mats, I can assume this is to allow new players to experience professions, the NPCs are nearly always located right next to a profession trainer.