This won't be very long it's quite simple, use your retainers if you have the full version.
Your retainers are your Gil lifeline! Make sure to always be selling items from your retainer on the market board.
Try not to sell things that are 1 Gil and sell every three weeks. It's not a very good and efficient way to make Gil.
Your retainers should as well always be on a venture, you can obtain them from beast tribe quests, leve quests or your grand company for seals.
When your retainer comes back it will bring materias dyes and Gil bags. Sometimes they will come back with furnishings that sell for a decent price as well. It depends on what type of venture you release them on.
Don't forget, there is a tiny button to check and compare prices of the item you want to sell on the board. This comes in very handy.
Due to the market board constantly changing you want to adjust prices constantly and never go to cheap with selling items, go one Gil lower than the one before or the same price.