So the Chagosian situation didn't have individuals buying land, it was a state (the UK) insisting on a deal to a much less powerful nation and then evicting all the citizens by force.
It's relatively clear from the wikipedia page (https://en.wikipedia.org/wiki/Chagos_Archipelago_sovereignty...) that the government of Mauritius were under the impression that their being granted independence from the UK depended on their agreeing to hand over the Chagos Islands
the key quote from British records being
"in theory, there were a number of possibilities. The Premier and his colleagues could return to Mauritius either with Independence or without it. On the Defence point, Diego Garcia could either be detached by order in Council or with the agreement of the Premier and his colleagues. The best solution of all might be Independence and detachment by agreement, although he could not of course commit the Colonial Secretary at this point."
It's relatively clear from the wikipedia page (https://en.wikipedia.org/wiki/Chagos_Archipelago_sovereignty...) that the government of Mauritius were under the impression that their being granted independence from the UK depended on their agreeing to hand over the Chagos Islands
the key quote from British records being
"in theory, there were a number of possibilities. The Premier and his colleagues could return to Mauritius either with Independence or without it. On the Defence point, Diego Garcia could either be detached by order in Council or with the agreement of the Premier and his colleagues. The best solution of all might be Independence and detachment by agreement, although he could not of course commit the Colonial Secretary at this point."
To quote the BBC (https://www.bbc.co.uk/news/uk-48371388) "Mauritius says it was forced to give up the Indian Ocean group"
The UK has a long history of gunboat diplomacy, this is just one example I would suggest.
As to the "huge number of abstentions", the vote was 116-6 of those that voted, not exactly a close outcome...