When they accept the amount that you offer. This is a voluntary transaction. You are paying money for a settlement. A large amount of money.
But that only applies if you barter with the ruler of the faction. If you barter with a vessel it is not the case.
There are kingdoms. That is a fact. All of these have a council advising the ruler.
A lord gets a fief when it is granted by the ruler who either listens to the council or not. If he doesn't listen they get angry.
If you are a vessel and sell your fief, you are in fact overuling a decision of your ruler.
If you are ruler and sell a fief you may be acting within your authority as a ruler, but you should still piss of your council. (Though that does not seem to happen in the game)
So if you are the one buying a fief from one of these two possiblities, you ought to be a very very very good trader
Typically when you buy a settlement, the head of the clan of the settlement does the transaction.
In a setting like Bannerlord, the head of the clan gets the final authority. Bannerlord is not a setting where you would see a modern democracy. It's a setting where a head has the final say - it's an autocracy pretty much and that's how this time period was run.
Is bartering a fief only possible with the kingdoms ruler? As I said, there are kingdoms, that is a fact.
I am well aware, that Bannerlord is not a setting featuring a modern democracy. Noone is talking about democracy.
But the clan leader is not the highest entity in the respective factions, it is the ruler of said factions.
Maybe trading fiefs should be handled differently depending on whether you are selling or buying, whether you are trading within your faction our between factions and whether the trade is part of a joining offer or not.