Simplest things. Not the fastest money makers tho would be

Making sure to do retainer ventures. Quick exploration frequently brings junk but sometimes your retainer comes back with something worth a few hundred K

Selling grade 6 materia. Do this by either farming clusters/crystals and exchanging them or they actually drop randomly in expert roulette duties. If you are farming duties you can burn all those extra tomestones on crafting materials. The key thing though when doing this is to not flood the market

Find a group and do some lv 70 treasure maps. Most tend to go with map owner rolls need on rollable drops but there's a lot of basic drops that auto distribute and bonus look if you open a portal. I decent night of treasure maps might take you a few days to sell off everything if you only have 2 retainers