It would help out servers that use the shopkeepers instead of other economy plugins. Think of huge quantities of farm items, or an inventory full of cobblestone. Currently it takes about 30 seconds to get to 1500 items.
A bit more difficult to implement, would be the same for the buy menu. Pressing down while the counter is at 1 will select the max quantity that could fit in your inventory.
Though I don't think that's very viable as I think the maximum a player can hold is only checked when they buy something, to see if it fits in inventory.