There are no government officials at the border monitoring or controlling what goods and services cross the borders. There are no customs agents at the borders collecting taxes. States and cities do not wage trade wars against each other. No one paces the floor worrying about the trade deficits between states and cities.
It didn't have to be that way. If the Framers had vested in the state and local governments the power to control and regulate trade, Americans would be living under the same type of system in which they live internationally. Each state would be imposing protective tariffs to protect inside-the-state businesses. There would be trade wars between the states and cities. Mainstream economic commentators would be sleepless with anxiety over the trade deficits between states and localities.
Free trade within the several states and localities is one of the major reasons for why the United States has always been so much more prosperous than other countries. That's because people raise their standards of living through the simple act of trade.
In every trade, both sides benefit. That's because each of the traders is giving up something he values less for something he values more.