magnanimousCynic and Frizze make some good suggestions. I've already been selling excess GC seals in the form of craft mats, so I can vouch for that bringing in a bit of gil.

I'd like to add though that you don't need to max out all craft and gather classes at 60 to profit from them still. I've made ~1M gil so far just selling excess products and materials from Leatherer, Armorer, and Miner that I'm leveling. They're all still in the 30-40 range. They started profiting around 25 or so, when most of the material products and inputs ended up being things not sold by NPCs, unlike the lower level stuff. It doesn't take a lot of time to reach that point, and when you do, you can essentially level the class while making money. Sure, a 60 Omni-crafter can rake it millions of gil with less effort, but it takes a lot of work to reach that point. Until then, a fledgling crafter's still gotta eat! (and decorate apartment). The game economy is friendly even to middling crafters.

For some like Leather and Armorer, refined materials like ingots and leather sell reasonably well on the marketboard. Crafted gear is hit or miss, but some of the pieces sell quite well. With the gathering professions, you can obviously just sell raw materials, but you might as well co-level a crafter with it and sell refined materials for a little more profit. Leveling in a profitable manner isn't as efficient as HQ turn-ins, but not all of us have millions of gil to throw at a problem.