This idea stems from another mmo where guild members can "sell" or deposit supplies to a vendor npc and guild members can "buy" or withdraw those supplies. The guild master can put a cap on how much of what item you can withdraw per day. You can also implement it to be disabled during off-woe hours and enable it an hour before woe.
The vendor npc should be initialized with two arrays. One array to store the item ID with the stack amount. and one array to store the Item ID and the cap amount.