It doesn't make sense at all as a perk but it happened in history many times and should be a normal part of gameplay to exchange fiefs with monetary compensation or other things a lord might want (like marriage) or to offer when trying to recruit lords to your faction.
Maybe keep it as a perk for the ability to outright buy a fief for just money but trading them for other equally important things should be enabled by default.
It could even enable a Normandy-like situation, say you are independent and making a living hell for a faction so they eventually get tired of trying to fight you and offer a fief in their faction for you to shut up and start terrorizing their enemies instead.
You are talking about super rare cases where people may have sold fiefs because they couldn't manage the fief or it was too poor or some other abnormal circumstance. No family is going to sell their only fief (or one of the very limited fiefs they have) to someone else, especially if that person is in another kingdom. I can see trading fiefs with people in your own kingdom, I can see offering fiefs as part as an alliance or marriage agreement or something, but not selling fiefs to some nobody or some potential enemy.