Not being refurbished is one condition by which an item may not be able to be traded. Refurbishment is accomplished through the Locker screen (L). To refurbish an item, open the Locker (L). Select the item in question, narrowing by category with the drop down box (weapon, clothing, vehicle, symbol, etc.). Edit the item using the Edit Item button within the Locker screen. Choose "Refurbish" from the buttons available while editing an item. There is a small fee to refurbish the item but this will make the item tradeable, assuming there are no other constraints to trading the item.
The item can then be traded via email or Marketplace.