Most of the examples have to do with the United States, like Alaska, the Louisiana Purchase, etc. What are some purchases of land in history that involved other countries?
Wikipedia has an excellently precise "List of territory purchased by a sovereign nation from another sovereign nation". All the examples given are since 1800, and it's worth noting that a few of them were linked to peace treaties after a war, not straightforward financial transactions.
(Since the European expansion in the fifteenth century, there have also been innumerable "colonial purchases" of land, often more or less forced transactions, taking place without much legal formality, and often without one side thinking they were actually selling land. I won't try and enumerate those!)
@Luiz notes in the comments that Bombay (Mumbai) was given as a dowry, as was Tangier; both were transferred as part of the 1661 Anglo-Portuguese Marriage Treaty, and could be seen as sort of a purchase (given the alternative might have been a lot more cash).
Similarly, Orkney and Shetland were pledged as security (ie, mortgaged) by Norway to Scotland in 1468; again, this was linked to a dowry, that of Princess Margaret, married to James III of Scotland. The dowry was never paid, so the islands passed to Scotland a few years later. (Norway did apparently attempt to redeem the debt later, but were rebuffed). Again, while not a direct cash transaction, it was a transfer of land in lieu of a cash payment.
For an older example (14th century), you might look up on Wikipedia Christopher II of Denmark, who mortgaged huge chunks of Denmark, and his successor Valdemar IV, who redeemed some of those mortgages. (The resolution of some of those mortgages also led in part to the complexities of the Schleswig-Holstein question centuries later.)
I rather suspect you can find many medieval instances in which cities paid to be granted charters that made them virtually independent city-states. There are also plenty of situations in which a monarch cancelled taxes (sometimes permanently) to secure election of his heir. This amounts to buying a country, in a way.
In 1860 France purchased Nice and Savoy from Piedmont in exchange for French military assistance in conquering other portions of northern and central Italy.