You can trade items by either selling them to the shop, mailing them to a friend or auctioning the item in the auction house.

You can mail items to others regardless of the location you are in, however to sell or auction an item you must be in a location that has trading facilities. If you do not see such a location in the map (indicated by a little icon at the top right of the location's square), then you must yet to unlock it or perhaps need to travel to the previous map to access such a location.