Greenland is not for sale; it is no longer a Danish colony, but part of the Danish rigsfælleskab - a word that is impossible to translate into one English word. It means that the two countries are close, Greenland has home rule, but receives help from Denmark, and the Danish Royal Family still visit Greenland.
The Yanks pulled a similar stunt years ago, demanding a U.S. Air Base at Thule on Greenland, which they got, much to the Greenlanders dismay. However, legally, selling or buying a whole country is a completely different matter, and not very likely to happen.
Davidhs, many Danes are sick and tired of the USA and I very much doubt that Greenland is prepared to sell themselves either.