[justshutupandtakeit]

That is not an accurate understanding of the meaning of this phrase. It is speaking to the Territory of the United States not yet formed into States i.e. the Northwest Territory which later became Ohio, Indiana, Illinois, Michigan etc.

It is not speaking about already formed states but territories. Certainly you do not believe that States are the property of the United States do you? By that interpretation the Congress could have sold South Carolina to France or Spain. Such an idea is ridiculous on its face.

In order for the suicidal act of secession to be legal the constitution would have to be amended since neither States nor Congress can legislate any changes in it.

[Poohbah]

Actually, the key point was the disposition of Federal properties within the seceding states. The war only started when troops from South Carolina opened fire on Fort Sumter.

[Libertarianize the GOP]

It is true that there is no provision for kicking a State out or selling it to another Country, but all the legitimate property claims and powers of the United States could be negotiated in allowing any State to secede. The power is there and nothing in the Constitution prevents such an outcome.