Well anything that is that "easy to gather" is bound to have loads of competition, which means lower prices and no "loads of gil" for you. Sometimes it's more profitable to "mine" the NPC merchant right next to the market board. Just find your opportunity and take advantage of it. The market is constantly changing, so it's "never the same river twice."

If you want mind-numbingly easy, I'd recommend looking into cul desynth. At the higher levels, most of the fish give 1-6 crystals per desynth, and most of the fish are dirt cheap. People always need crystals, so there's always a demand.