Important Note: The Market no longer appears in War for the Overworld. It was removed from the game after this article was created.
The various weapons and armaments that your minions create within the Foundry have to go somewhere... thankfully there are a few business-minded Gnarlings who will be happy to set up shop within your dungeon and sell the crafted works from both the Foundry and Alchemy Lab to your minions.
The Market is one of the rooms available by mixing the Vein of Sloth and the Vein of Greed.