A great way to make money is to hold onto those Beastmen's Seals!

If you're choosing between an Earth Crystal and a dinky old Beastman's Seal which no NPC will buy... don't make my mistake (I learned around my late teens to keep them). Keep everyone you get, cast lots on them in parties if they let you. Though they won't make you any instant cash, later on you can use them to trade for certain orbs which let you take part in BCNM (Burning Circle Notorious Monster) battles. These are fights where you and your party of 3-6 (depending on level and area) fight 10-12 NMs for awesome drops.

I wouldn't recommend BCNM20 or 30, if you can, take BCNM40's. You can get Phalanx (170,000 gil on Ragnarok), Erase (300,000 gil on Ragnarok), Utsusemi: Ni (180,000 gil on Ragnarok) and dozens of other lesser items from these battles.