[Phlyer]

. . .Federal Government has 18 enumerated powers, where it can exercise supremacy of its laws over the states.

Yes, and one of those enumerated powers is "to exercise like Authority over all Places purchased by the Consent of the Legislature of the State in which the Same shall be, for the Erection of Forts, Magazines, Arsenals, . . ."

What happens to property the feds are using in a state, like military bases or the federal courthouse or Fort Knox if a state demands the feds get out?

Listed within that enumerated power is that the State needs to consent (at some point, perhaps well in the past), and that the land used must be purchased from the States. There is a question on whether that consent can be withdrawn - and on the basis of that ambiguity the statist judges would surely rule that any attempt to regain that land would be illegal unless the Federal government consented to release it.

And since it was purchased, I think it is reasonable that the Federal government would expect compensation. But I think there is a very credible Constitutional process where South Carolina could have informed the Federal government that they no longer consented to Federal troops in Fort Sumter and were returning the purchase price. After that, it would have been the Federal troops who were in violation of the Constitution, not the Confederate forces.

A similar process would apply to any other Federal land. Of course, in many cases the Federal government just seizes the land (claiming it is for some noble purpose like a National Monument).

[Jonty30]

The Constitution does forbid just seizing land, obligating the government to providing reasonable compensation.

I trust if this were to happen, that there would be haggling back and fort about the just compensation before a final price were to be given.

[x]

There is a question on whether that consent can be withdrawn - and on the basis of that ambiguity the statist judges would surely rule that any attempt to regain that land would be illegal unless the Federal government consented to release it.

Any judge would agree with that. You can't have a state suddenly deciding to take over all courthouses, post offices, and military bases with its borders. As it is, though, some closed military bases have been turned over to states and cities.