Help me out here. I recall reading, probably in The Politically Incorrect Guide to the Civil War, that the shots fired at Ft. Sumter had to do with the fact that Union troops had ensconced themselves there and refused to evacuate, defying Confederate insistence that they were occupying the sovereign soil of the newly declared secessionists.
Your statement about ships and export tariffs has me nonplussed.
I think there was a combination of the two. You are correct about Union troops. The issue remains a States Rights vs Centralized Federal Government. That issue has evolved to include Marxism, a later development of centralized control theory in government.