Regarding making gil in this game;

Crafting and gathering. any and all HQ precursors that you can sell will always sell at a premium and sell reasonably well. By pre-cursor, I mean raw materials and/or basic crafted items used as ingredients in other recipes. The best two examples are lumber and ingots. that So you might sell ores or logs/branches - for example, but depending on the items, you may find that the finished lumber sells better than the logs (so you can always level your CRP and use a simple macro to HQ the lumber. The same is true of blacksmith and armorer, make ingots, plates, nails and seel the HQ ones. These are relatively easy to HQ once you are a few levels beyond the level of the recipe. Leather is also good in the same manner, and there are good items for weaver too. To a great extent your ability to make gil this way depends on the time you are willing to put into it.

Retainers can be a good source, they occasionally bring home something rare from the ventures that you send them on (depends on their level and ilvl), sometimes it's something very rare that you may be able to sell for millions of gil on the market board. They can also be used to gather raw materials as well as shards and the like.

Finally, the Challenge log is a reliable source of gil, not a huge source, but it's kind of an incidental bonus, so pay attention to it, you might find that with a little change to your routine, you can gain additional, 'free' gil.