First you need a retainer. Once you have one, you can go to any bell (they show up on maps) and give them your items that you want to sell (through the exchange menu).
Once they have the items you want to put up for sale, drop back to the retainer menu and you will have an option to manage their bazaar.
Select that, and you will see a list similar to your normal items list (as opposed to an item exchange window).
Select an item you want to sell, and choose the Transaction menu choice.
Here you can choose the price, quantity, and sale type (always choose Unit).
Confirm the settings, and you are almost there.
Now the part that wasn't covered above is that you have to place your retainer on a floor in the Market Wards. This can be done by going to the floor you want in a market ward, and summoning your retainer (when you are on the floor, a special menu option will appear for Summoning Retainer, select that to get them there).
Once your retainer is summoned to a floor, they *should* stay there until all of their items are sold, or 7 real-life days, which ever happens first.
If you interact with your retainer once a week, they will keep their location on the floor.
You can perform all actions you would with your retainer in a market ward (the above steps I gave you work better in the Market Ward rather then at a bell, simply because you still need to put them on the floor of a ward anyways).
If you summon your retainer at a bell, or a different Market ward (e.g. Ul'Dah, Gridania, Limsa), they will move to that location (e.g. when summoned at a bell, they are pulled off the market ward and put in limbo, otherwise if they are summoned at a different ward and/or floor, they will stay there).
To retrieve your money (or items if you request to purchase), simply go to your retainer, and choose to exchange. There will be a tab on your retainer for currency. Select that tab, select the money, and then transfer it to you.
Sorry if this is confusing, I am not that good at walk-throughs:P.