Since you claim that the United States was not created as a "prepetual union" under the Constitution, what exactly prevents a 2/3 majority of the Congress and a 3/4 majority of the other states from selling any given state to a foreign power even if that state does not wish to be sold?
The power was never given to the Federal Government to conduct that kind of sale.
If the power is not in there and enumerated, it doesn't exist.
That's what prevents them from doing it.