Really, a country can do that?

It seems the Maldives are doing a new stunt in the world... Expanding their land to save themselves. Is that legally possible to buy land from another country and take over it as your own sovereign land, without proper treaty? Jeez, that sounds like Sealand all over again. Yes, Sealand is real, but it's an example of what goes wrong with international politics. Sealand