I can't speak for any other countries, but the French crown post-1000 was very often buying back land from the French nobility. Robert Fawtier has a lengthy list of specific transactions in The Capetian Kings of France: Monarchy & Nation 987-1328.

For some context and backstory (and here I am specifically referring to and paraphrasing chapter 18 of Chris Wickham's The Inheritance Of Rome: Illuminating The Dark Ages 400-100, which is imo is a very accessible overview of the post-Roman territories in the early medieval era):

The late Carolingian era was, to put it mildly, fairly a disaster for the royals. In the late 880s, the Carolingian (descending from Charlemagne) line was facing a succession crisis: The only surviving heir to the West Francian throne was a small child, Charles the Simple, and the region was suffering from Viking attacks. Odo, the Count of Paris, had had some victories in driving off the Vikings before, and the nobility chose him to lead in 888, though the southern nobility was apparently hesitant about the choice. (Wickham speculates that this had to do with the fact that Odo was from a rival family and not Carolingian, though I'm not sure I buy that.) His success, though, soon dried up, and by 893 the archbishop of Reims publicly declared Charles the Simple king, and civil war soon followed. In return for Odo's abdication, Charles allowed him to keep key lands in the Parisian region; this, along with other opportunistic advances by other counts (a father and son of Vermandois), who also managed to keep parts of the royal heartland, severely reduced the royal domain near Paris.

Charles the Simple was thus in 898 made king of a post-civil war state and in a very precarious position. The central authority of the king, already nebulous enough, seems to have collapsed in this era; in East Francia and Italy, rulers were able to freely select counts and dukes and even reposition them and send them to different dominions; the French crown loses this ability in this time. (Charles the Fat had used this power in West Francia in the early 880s, so it was clearly a recent development.) Wickham specifically notes that no tenth century king had any effect on any succession of any county or duchy; these were now tending to become hereditary and beyond the control of the monarchy, except by direct invasion. I'm unsure if this relinquishing of power was promised away in an attempt to "buy off" the nobility and earn their favour (Charles must have known that he was in a very bad spot), but nevertheless the nobility does not seem to have taken to Charles, as (in an account ascribed to Flodoard of Reims) "almost all of the counts of West Francia" rose up in rebellion in 920; and, to make a long story short, Charles eventually dies in prison in 929.

So, you have a post-civil war state, where a weak king, coming in off of a succession crisis, loses control of his counts and dukes, and has promised lands around the Parisian heartland away. Yikes!

But in comes the new Capet dynasty with a new strategy (though it would be a century or two before it really kicks into gear): Buying parcels of land from those counts and dukes and expanding the royal domain piece by piece.

From Fawtier:

The thirteenth-century Capetians increasingly made use of purchase as a means of adding lordships of all sizes, and even small parcels of land, to their family possessions... When Count John of Beaumont-sur-Oise died without direct heirs, his succession was disputed between Thibaud d'Ulli, son of his cousin-german Ives de Beaumont, and Ives' two sisters, Beatrice and Mary. Thibaud was award the fief by a decision of the royal court in 1223; it may have been in token of gratitude that he shortly afterwards sold a considerable part of the county to Philip Augustus for 7000 livres parisis.

And a longer passage:

Other instances can be given. In June 1224, Louis VIII bought the cast of Montreuil from Guillaume de Masnières for 200 livres parisis and in December he agreed to pay an annual rent of 100 livres tournois to Galeran d'Ivri, Viscount of Meulan, for his property at Beaufort in Anjou. St. Louis acquired the county of Mâcon in 1239 for 10,000 livres tournois and a life-rent of 1000 livres... Philip III used much the same methods in 1281 when he paid Count Arnoux III 3000 livres tournois and a life-rent of 1000 livres for his county of Guines... Count Renaud of Guelders surrendered his lands in Normandy, including the port of Harfleur, in return for a life pension equal to their revenues; and Gui de Mauléon, knight, exchanged the castle and barony of Montmorillon for a cash payment of 1200 livres tournois and an annual rent of 130 livres.

Philip the Fair made many purchases. He bought Edward I of England's rights in Quercy in 1289, giving in exchange an annual rent of 3000 livres. In 1286 he bought the county of Chartres; in 1291, Beaugenci; in 1293, the fief of Montpellieret... in 1302, the viscounties of Lomagne and Auvillars; in 1306, the viscounty of La Soule... An even more important acquisition, that of the county of Burgundy, was virtually a purchase also. The consent of Count Otho IV to the marriage of Joan of Burgundy and the king's second son Philip (which gave the latter immediate possession of the county) was only secured by a payment of 100,000 livres tournois and a life-rent of 10,000 livres.

I could go on. The point is, quite a bit of the French royal domain was acquired through buying land off of nobles, often in exchange for pensions and life-rents and huge sums of cash up-front. This, of course, leads to a snowball effect where the crown has more resources, so it buys more land and expands its power, and so it has more resources, etc. (Which is why it takes so long for it to really start accelerating at a noteworthy pace; you may have noticed the time jump from the late 900s to suddenly 1200.)

I can't speak for other countries at the time, but land transactions were extremely common between the French crown and the French nobility, often on the side of the French crown purchasing.