So move the trait WAY down the ladder, like to 100 or so -
But nobles will only sell a property that isn't part of their native faction. So Sturgia will happily sell you, for a huge fortune, the holdings in Vlandia it captured. Owning a property gets you 1/2 the relationship hit that conquering gets it, so whatever noble originally owned your new town is always going to be pissed at you and that faction is always likely to try and get it back.
Nobles selling actual towns, villages and castles - the basis of their factions control over a region and the basis of a whole clans noble title, power, source of income and authority, is kinda silly. An exception would be if you're already pledged to a faction you should be able to trade towns/castles and buy villages with an influence expenditure and a positive relationship with the faction ruler.
The idea of a faction selling away part of its core territory was always a bit wonky. Someone mentioned the Louisiana Purchase - yeah, sure, they sold a chunk of land they'd claimed, never colonized and were not in a place to defend or settle. If you, as some trader from another land, had gone to the Duke of Burgundy in 1200 and said 'Hey, I'll give you a bunch of horses, these 30 carts of blood stained looter gear and 2 million Franc for Burgundy' at absolute best, absolute level best, you'd be laughed at. He might just take your 'deal' then laugh all the way to Paris as King Phillip the whatever sent his armies to kill you for try to steal part of his kingdom because only the King gets you make you Duke of Burgundy - because Burgundy is part of his Kingdom.
I'd rather see a deeper trade mechanic - have 4 or 5 or 6 shops in a town (with the right perks), be able to buy faction specific influence you can use to get nobles in a faction to do what you want. Hell, get a right of Monopoly (selling monopolies was a popular way for nobles to make money from wealthy merchants).
Just buying kingdoms though seems... I dunno. Really gamey. Yes it's a game and such but being an untitled foreigner and just buying big chunks of a kingdom has always seemed a bit odd.