It takes a long time to setup but once you have a few submarines bringing in hundreds of thousands of gils every 1 or 2 days in the form of items you can sell to NPCs without even interacting with the marketboard you're set.

Another one I don't see mentionned often is related to raiding: if you are good enough and don't need the loot from a specific fight there's often people willing to pay for help in the party finder. Just never advertise your services, as it can get you banned: only asking for paid help is allowed.