The fastest possible way to make money would be to send out fake emails to rich people, and pray to God they fall for it, thereby downloading a keylogger or involuntarily giving you their password. Unfortunately, that's totally against the rules, and hopefully against the moral standards you (should) hold. Efficient ways include crafting nature runes, mining and smithing steel bars, merchanting, selling logs and fish, and (to a lesser extent) combat. Although clues can really add up, especially if they're level 3 clues.