Most vendors will have a 2nd tab at the top of the screen marked sell. By clicking it you will open up your inventory and be able to sell items to these vendors. The Bazar will also allow you to sell almost all of your items if they do not have a "no sell no auction" tab attached to them. To sell your item, clik on the item you wish to sell and press sell under the portrait screen to the left of the item's name.