Duty free is essentially tax free...not just local sales tax. Many items get taxed by the government when imported into the country, then you add state tax and local taxes. On some items it could be 30% or more cheaper being duty-free. Most popular items are cigarettes and alcohol though someone who uses them often could give more examples.
Tax for a carton of cigarettes in New York 27.50
Tax in South Carolina 70cents
Depends where you live on how much savings you would see.