[kfcm]

That's incorrect about the Constitution. While it does not lay down specifics, it does give Congress the power to regulate interstate commerce. Known as the Commerce Clause, it's Article I, Section 8, Clause 3: "To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;"

[rayiner]

The Commerce Clause precludes states from erecting trade barriers between each other. While Congress could in theory do so, (1) it's never to my knowledge ever used its Commerce Clause powers in that way, and (2) provisions like the Privileges and Immunities Clause creates independent prohibitions on erecting barriers between states.

[ashelmire]

Yet... alcohol can not be freely moved between states. There are clearly cases where interstate trade is prohibited.