Here’s the actual Resolution of 1780.
“That the unappropriated lands that may be ceded or relinquished to the United States, by any particular states ... shall be disposed of for the common benefit of the United States...
http://www.minnesotalegalhistoryproject.org/assets/1780%20Resolution%20on%20Public%20Lands.pdf
Couple comments, though I don’t claim to be an expert in this area.
“Disposed of for the common benefit of the United States” means, I believe, that the price of this land, when sold into private ownership, goes into the Treasury of the United States. I didn’t see anywhere in the Resolution where it stated that title to these “unappropriated lands” was to be handed over to the new states so they could sell it off. That wouldn’t be “for the common benefit of the United States,” would it? But it’s possible I’m misunderstanding something.
Also, note that the Resolution refers to “lands that may be ceded or relinquished to the United States, by any particular states.” The lands acquired later by the US under the Constitution west of the Mississippi were not “ceded ... by any particular states.” They were acquired by the US through treaties with France, Mexico and Great Britain, and by treaties with the Indian tribes.
“Disposed of for the common benefit of the United States” means, I believe, that the granting of land to the railroads should be viewed as being for the common good. I have no problem with that.
It’s the retention of the rest of the territorial lands that I see as illegal.