The idea behind the infinite gil trick is to buy raw materials, refine them into a finished product, and sell that product back to a shop for a profit. For raw materials that are readily available and that can be purchased from the shops themselves, this means that you can continue buying and selling items over and over again, until you have as much gil you want.
Its hard to call this a cheat given that these are options readily available and exploitable in the game. Maybe it should just be considered a clever use of game mechanics? (Or maybe a clever abuse of game mechanics…)
Purchase 100 
Tent items from one of the shops, either when you open the shop directly or by utilizing 
Call Shop to pull open a shop anywhere. Use the Recov Med-RF ability (which Carbuncle can learn) to turn those Tents into 
Mega-Potions.
100 Tents will cost you 100,000 gil. They will refine into 25 Mega-Potions which sell for 125,000 gil each which means you will earn 25,000 gil each time you complete this process.
There are a number of steps you can take to increase the speed an efficiency of this process. These all have to do with specific abilities that Tonberry can learn, including:
Call Shop: Allow you to access any of the shops in the game at any time by selecting Call Shop from the ability menu in the main game screen.Familiar: Opens up new items for purchase at some of the shops. Namely, the shop in Esthar (Johnny’s Shop) will now sell you Cottage which can also be refined into Mega-Potions using Recov Med RF so you can convert more items at the same timeHaggle: Allows you to purchase both Tents and Cottages at a discounted price.Sell-High: Allows you to sell Mega-Potions back to shops at a higher price.
Using these advanced steps, each time you buy raw materials and resell the refined products back to the shop, you will bump your profits from a measly 25,000 gil up to a total profit of 562,400 gil each time you repeat the process.
