I think that Trade XP should be proportional to the profit made, and not only gained when there are none of the sold widgets in stock at the station. Or is there a good reason for the current system? I just think that would be more logical.

Thoughts?

the whole trade XP system is in need of moderate to severe repair, but then again IMO the whole economy needs work.

The reason it's the way it is can be traced to a very simple reason. It's something of a kludge.