Consumable items could be sold straight from a vendor called a Marketplace Trader, or crafted at a consumables crafter of some sort, similar to the scrolls of insight currently in-game.
Coupled with focii/staff bonuses, this could add an extra level of depth to the game's mechanics.
Some examples of consumables I can think of currently are:
-Adrenal Boost:
Attack damage -20%
Adrenaline gained +20%
Duration: 2 minutes
-Mind Boost:
Maximum Energy +20
Armor -10
Duration: 5 minutes
-Quickshot Elixir:
Arrow flight time -50%
Arrow accuracy -10%
Duration: 2 minutes
These are just examples, as I've not gone through all the permutations of what could possibly imbalance some classes, but with some tweaking I think that this could make both a viable gold sink as well as an interesting addition to gameplay

Thanks, and any suggestions or comments are welcome.