Easy Money:
In order to get rid of unnecessary items stored in your chests, you can trade them with a goblin in the fortress for Mithrill.

Go into the room where the wizard is and go out the door across the room. Walk down a little and adjust your map so that you are looking directly north.

You will see the goblin in front of you and he will ask for some item in exchange for Mithrill.

Every time you trade with him he gives you more Mithrill.