The Cotton Robe Synthesis Trick is a very basic trick that takes advantage of some basic issues with buying and selling items. There are raw materials that you can purchase, synthesize and then resell for more money than the initial raw materials were worth.
You can use this trick to make easy gil early on in the game. Here is how it is done:
Purchase:
Wrist (130 gil)
Steepled Hat (260 gil)
Synthesize into:
Cotton Robe (cost 1,000 gil to synthesize)
Sell the Cotton Robe for 2,000 gil netting you 610 gil for each Cotton Robe synthesized.
There are numerous points in the game where you can utilize the Cotton Robe Synthesis Trick including:
This trick is incredibly useful for earning a ton of gil fast within Final Fantasy IX.