A quick way to make money is via the infinite gil trick. You need Carbuncle with the "Recove Med-RF" ability learned (Tonberry with Call Shop, Haggle, Sell High, and Familiar learned helps a lot as well). Then do this:
Go to Johnny's Shop in Esthar. It may be closed at first, but enter repeatedly and you will eventually be able to shop there. Once visited, it will appear on the Call Shop list as "Esthar Shop!!!"
Buy 100 Cottages and 100 Tents. Johnny's Shop is the only place that sells Cottages (If you missed it or are not at that point in the game yet, use another other shop that sells Tents).
Using Recov Med-RF, refine the 100 Tents into 25 Mega-Potions and the 100 Cottages into 50 Mega-Potions.
Sell the Mega-Potions for a huge profit. Having learned the Haggle and Sell High abilities will increase your profit margin from this trick.
Repeat as long as desired.
