I had an epiphany while killing brutal black dragons. They dropped 3 mithril bars each, and i thought, being able to note these from my inventory, would be incredibly helpful for both me, and the people wanting to buy them for less than 75k each. If you are unfamiliar with the concept of notepaper, you can find information about it on the rs3 wiki.
As to how it would be implemented, I would leave that for the experts. Source could be anything from voting, monster drops or PKP.
Feel free to express your opinions below