There is something strange when you trade items. When you start trading, the "all" button is selected. When you click on an item in your inventory, it will disappear in your inventory and show up in the central part on your side (sorry, not sure how it is called)
If you click on the "junk" button, absolutely all items are shown. If you click on an item that can not be seen in the "all" button (like keys or books) they can be seen in your inventory and in the central part. You can sell these things without problems.
Prof. Dr. Dr. Mad S. Tist
World leading expert of artificial stupidity. Because there are too many people who work on artificial intelligence already
Prof. Dr. Dr. Mad S. Tist 
