A player can sell any tradeable item out of their bazaar by setting the price per item. Goods are sold individually, not by the stack, so price your items accordingly.
To make a purchase out of someone's bazaar, /check the player and select the option to view their bazaar.
All purchases are final, so buy with caution. It is often a good idea to check the Auction House or NPC Merchant prices for that item before buying out of a bazaar.
To sell your items in your bazaar, go to the second page of your main menu (on PC by pressing the "-" on the number pad twice; or by pushing the "Square" Button twice for PS2 players, or by pushing the "X" button twice on 360) and choose the "Bazaar" option.
You can choose to either "Sell Item", or edit your "Bazaar Comment". This comment is seen when others /check you. A number of people like to put sales comments there, although you can put any comment there.
If you choose "Sell Items" it will bring up a list of all the items in your Inventory. Items that you can't bazaar are greyed out. Choose the item you want to sell and enter a sell price. If you have a stackable item, the price you enter will be how much one of the items will cost.
Example: You have 8 Fire Crystals and you enter a price of 700 gil. Someone checks your bazaar and can buy up to 8 of your Fire Crystals at 700 gil each.
When you're editing your bazaar, it becomes unavailable to other players. If you are muling you may want to make sure that you bazaar icon is visible (See: Player Icon Priority)
There is no bazaar tax for making a purchase out of someone's bazaar in certain cities.
As of Version Update:01/26/2020: