First find someone who wants to buy the item or make a thread in the commerce forum. Someone should contact you and you can work on the price. If it's a big deal, use a middle, or else they send first.
All you have to do is go to your inventory and check the box next to what you want to sell. In the drop down box, go to "make over". Enter the name of the person or faction you want to sell the item to and make it over.