Ex primal mats and music, materia, crafting mats.
I made the vast majority of my money through selling things stand it's peak. I remember specifically Kingcakes and Luminous Fibers making me stupid amounts of Gil.

It's really all about jumping on high selling items when they're new. Primal music goes for millions the first few days, and people WILL buy it. Jump on that.

Oh, and don't forget tome mats. That's at least 200k per 2k tomes.