In the 50+ dungeons, you sometimes see tradable loot. Some of them drop crafting mats that arent easy to get elsewhere. And most of the 50-60 dungeons have a minion drop that can be sold. The gear itself cant really be sold, but if you have time and wanna put in some work money can potentially be made there too. If youve leveled a crafting class or two, you can do a quest to learn how to break gear down to some of its component mats - most are sellable. And nothing is stopping you from spirit binding and converting to materia every piece of extra gear you get(though it would be a big time commitment). There might even be a few items sold by your grand company for seals that you can turn around and sell(expert delivery is a good source of seals if your rank is high enough to have it).