If no one has a resource, when trading it shouldn't tell you "Error: No one has that..."
It's not up to the AI running the game to tell players that no one has a resource requested. It's a players job to try to keep track. As it is, it can save a person from using (and wasting) a monopoly card... Right now, never play a monopoly card until you first try to make a trade for it. If it says no one has that resource, then you just saved your monopoly card for another time or resource.