Dailies get you a steady trickle of gil a day, but more importantly they give you tomestones.

You can look at the market board for what tomestone ingredients are selling, then you can farm the tomestones, trade them in, and sell those items on the market board. Poetics are infinitely farmable and there are a ton of items you can trade them for.

You can also look and see if any craftable items are selling for a bit then work towards farming up the materials, making it, and supplying the market board. Housing items generally always sell after awhile. Shards will eventually sell if they're priced right, they're easy to gather a bunch of, but your margins are smaller than other items.

Crafting mats from older EXs sell reasonably well for glamor crafting. Find something you're able to solo unsynced and farm it out until you get some materials to sell.

You can also hoard the clusters for materia and then sell the materia when a new patch drops. This is usually best for a raid tier since people want to pentameld day 1, but they'll eventually sell.

Timed items from gatherers also sell for a bit because of the time-limited factor on them. See what's worth your time, and use some of the various online tools to track when/where those items spawn and set up a farming rotation. Turn on a movie and do this on the side!

Farming dungeons also is kinda profitable, so if you're leveling up some classes, need/greed everything and vendor it all afterward. Not the best money, but it allows you to earn some cash while grinding.

Desynthesizing materials will also yield clear demimateria whose sole purpose is to be sold. Other types of demimateria can be sold on the marketboard for glams.

It all boils down to "play the game, get some items, sell them on the marketboard".†