BTW, while the whole concept is silly in the first place, basic Constitutional law would teach us that Montana is not the USA's to sell, because the USA is a creation of the states, not the other way around. If you want to make a business analogy, Montana owns a 1/50th share in a corporation known as the "USA".
And they'd need Montana's willingness and agreement to go in any case as no state can be removed from the Union without its own consent.
Well, the constitution could be changed.
I don't buy it. There is, to my knowledge, no part of the Constitution that basically says "these are the rules by which a State may leave the Union." Rather, the Constitution is a construct of a Union of States, and it has therefore been determined by no less than the US Supreme Court to not allow a State to leave said Union except through the consent of the States. This is not simply something you can alter in the Constitution by amendment, but rather extant constitutional law.
… the Constitution was ordained "to form a more perfect Union." It is difficult to convey the idea of indissoluble unity more clearly than by these words. What can be indissoluble if a perpetual Union, made more perfect, is not? … And we have already had occasion to remark at this term, that ... "without the States in union, there could be no such political body as the United States." … The Constitution, in all its provisions, looks to an indestructible Union, composed of indestructible States. When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration, or revocation, except through revolution, or through consent of the States.
I think it's fairly clear that the phrase "consent of the States"–along with everything else about indissolubility, indestructibility, finality, and perpetuity–means
all states, including the one in jeopardy of leaving the Union. It should therefore be considered constitutional law that a State cannot be kicked out of the Union by the other 49. Changing the Constitution would not un-write that constitutional law, and any amendment intended to contradict it would be determined inadmissible.