Typically, such an item has no purpose other than to complete the mission it is designed for. It cannot be sold to vendors or traded to other players, so it has no monetary value. If you have completed or abandoned the mission, you may delete the item.
If you find an item in your backpack labeled "not sellable, not tradeable", and you don't know its purpose, chances are, it's a mission item. Check your mission log for any active missions related to the item. Alternatively, search TaRapedia for the item to see which mission it belongs to.
For a list of mission items, see: Category: Mission items