FFXI Gil is the main currency in the world of Vana'diel. Adventurers need Final Fantasy XI money to update weapons and Armor, to purchase items, cover the cost of transportation, and other normal expenses. There are two ways to sell your goods to other players: The Auction Houses, a staple of the economy in Vana'diel, and the Bazaar system.
The Auction Houses
At the Auction houses, players submit for sale and set the price they wish to receive for the item. The first player to bid at or over that price wins the item.
Auction Houses are usually located in somewhere on each map of a major city. Most commonly they are placed in the middle of a Market. You will know you are at an Auction House when you see wooden counters and all the people standing around these counters. Sometimes you can also find Auction Houses without counters. In some cities, for example, at Jeuno, there are NPCs who you can talk to that will initiate an Auction House. A good way to tell if a NPC is being used as an Auction House is by checking the number of people around it. Usually large crowds don't all talk to NPCs unless it's an Auction House or an important mission or quest.
Sell your goods at the Auction House and make a profit. The downfall with selling goods though is that it takes time for people to spend FFXI gils buy your goods. You sell your goods based on the price history of that item and if you stay within the price stated in the history it should sell quickly. The price history of item can be found by trying to purchase the same item you are trying to sell. If an items sell for cheap FFXI gil in the price history and you stated higher later, it's hard to sell. When you go to purchase or bid you would select that item and then select its price history and then jot it down. Then you would go back and sell your item for the price that you wrote down and it will eventually sell. Of course, another quality is the ability to buy various equipment, usually are not sold in most stores. If you need a certain sword or armor, your best bet is to search the Auction House.
The Bazaar system
By utilizing the Bazaar system, players can set prices on items in their inventory to make FFXI gil. Nearby players can check the player's Bazaar, see what items they have for sale, and buy things immediately if they agree with the price. Make sure to try and haggle before committing to a purchase.
Instead of making FFXI gil in the game by yourself, you can also buy FFXI gil from other players or vendors. So you can easily purchase weapons or Armor to save time for other interesting adventures in the game.