Material traders would be a useful NPC (or NPCs) and a crown sink. You could have a material trader (0-4*) and a rare material trader (5*). Prices are set within a certain range, use the current market for starting prices, and let them fluctuate by supply/demand within the trader system. The buying price would obviously be lower then the selling price (say 20% lower). It would allow newer players that wish to sell their own mats as a base to price them around, and allow other players that just wish to buy/sell the mats fast a place to do it at a decent price... instead of 25 crowns for a 4* mat.

~Gwen