You can get Gil from quests.

You can also get Gil from doing the daily roulettes, and squeeze out more by signing up as the job that is required at time you queue.

Otherwise, your retainer is the interface to sell on the market board. You don't need to guess what something is valued at. There is a button to click which shows you what the item you want to sell is currently going for.

Even if you bought jet black dyes from mog station (and I don't even know if resell is prohibited), you can't flip that to Gil without a retainer.

No NPC character is going to items of any significant value from anything looted in your inventory.

Actually, there is another way. But you need a Culinarian that's up there. Either buy, or craft Coffee Biscuits and hand in Leve quests.

Beyond that though, I'm afraid to say you're going to have to change your way of thinking and approach to this game for ways to make money. Or be content with your decision to remain poor.