It would be super convenient but wouldn't people just over buy from the AH to store in NPC? Which i guess wouldn't be too bad but it might lead to dry spells as to when there would be stock in the AH since you'd be essentially locking in the price of dust in case prices go higher (unlikely but locking in a good price might be just enough incentive for dust supply to dry up).

Although that isn't a bad thing since its not like they're being stolen from since people selling them are getting their pay