Most of my gil came from mining/botany, if you get the latest maps for legendary items and go to town when items are brand new, you can usually make a mint over a couple of weeks before prices plummet. Another good (if temporary) source is crafting items only purchasable with tomes. When the items are new, you can make a killing in the first day, especially if you have the foresight to have a full 2000 tomes ready to turn in as soon as the servers come back up. Finally, items needed for each week's Fashion Report typically surge in price so if you can figure out in advance from the clues what the item will be you can be ready for that too. (Just don't go too crazy, since they'll be back to nothing in a week.)