Before deciding yourself on a method, dont forget to check the prices yourself at the market board, because they can be different on your server. For example the rank 6 materias that used to sell well on mine, for around 120k gils, have dropped down to 40k-60k since you can get back 40% of those already used on an item, and since hunts can give you 340 upgrades (more people doing them = more cracked cluster = more materias 6 on the market = prices going down).

Dont forget to consider the time needed for a method as well. If you take too much time leveling your fisher the prices might have changed in the end.

That being said i earned most of my gils reselling rank 6 materias, and Aquapolis/Uznair's drops. Some items found in the palace of the dead can reach a high price too.