r207, "sell" is a gross oversimplification of what would be a complex transaction.
Denmark has a legitimate colonial interest in Greenland, compounded by the fact that it has objectively been beneficial to Greenland itself. Denmark has told Greenland that it's free to pursue full independence if that's what it wants... but the free money stops if it does.
Denmark also has substantial state-held real estate holdings in Greenland that exist independently of the sovereignty issue. Denmark could let Greenland become fully independent without invalidating its ownership of its real estate holdings there. This isn't a controversial viewpoint, and the UN, US, EU, and NATO would ALL back it up 100%.
So, yes... if Greenland were to become fully independent of Denmark & voluntarily join the US, Denmark would have legitimate expectations that the US would reimburse it enough to make up for all the money it has spent in the past supporting Greenland... and the US would fully agree.
Greenland is also part of Denmark, so any transfer of sovereignty would also need democratic approval by Greenlanders to be acceptable and legitimate... with plenty of mitigations to protect the rights of Greenlanders who'd view its union with the US as abhorrent. A national referendum, being allowed to accept US citizenship while retaining Danish citizenship, along with the freedom to remain in Greenland as an expat Danish citizen & abundant financial compensation to enable Greenlanders to choose any option without being limited by personal financial constraints would be another essential part of it.
Incidentally, it would ALSO be an enormously better deal than residents of Hong Kong were offered.
The point being, nobody would be getting bought or sold. National sovereignty might change, but only as the outcome of a democratic process. The US has no interest in participating in a "hostile takeover".
In any society, you'll always have some people who'll oppose almost anything, and that's their right. But it's ALSO understood that a minority shouldn't be allowed to exercise unlimited veto power over a majority, as long as the aggrieved minority is compensated & made whole.
If one or more of Greenland's smaller municipalities were united in opposition, I'm sure the US would allow them to remain independent (or part of Denmark)... though I suspect, as autonomous city-states under partial Danish rule (but without most of their present subsidies), they'd lose their best & brightest to emigration to Denmark, the US, and/or the new US State of Greenland... because small indigenous settlements tend to end up being rather poor & unappealing to kids who grow up there & know what's available elsewhere.
It's not because countries like the US hate indigenous people, it's because traditional subsistence societies simply aren't economically viable in a globalized economy. You can invent a product (like casino gambiing), embrace globalization & aggressively develop resources like oil, minerals, commercial agriculture, etc,... or you can do nothing, then bitch & moan when your kids flee & never look back.