Basically any material increases in value as it is more refined. If you mine a Silver ore and try to sell it you get about 60 Tria, if you have the slills to refine it into ingots you get 169 Tria, so a player might buy silver ore from you for 100 Tria each, knowing after refining he gets 169 each, so gets 69 Tria profit for each ore without having to go out and mine it himself, and you get 30 Tria more than if you just sold it to a NPC. Further refining into Silver stock again increases the value. of course Gold is the ore with most value, so that's the one most purchased.
