I have a few things to add to this. I have often seen players with ads claiming they are future NT players and often those claims are clearly lies but maybe they fool the occasional noob. Nothing that can be done about it.
Worse there have been cases of NT coaches adding their own players to their NT rosters and selling them so that they can get a great price. There was a NT coach sacked for doing this a few seasons ago, I think from memory it was Pakistan but sorry to all Pakistan coaches if it wasn't.
I do have one suggestion albeit a bit extreme. You can temorarily edit your NT advertisement that will appear on all players.
You can add a PS, some thing like.
...has U21NT potential.
PS: If you just bought XXXXX XXXXXX under the belief that he was a future U21NT player I am sorry you have been scammed he will never make the NT and you should report the seller if he lied to you about it.
The current and then the new owner should see this and you will only need to keep it on for a few days. Just seeing it may put the seller off his dishonest advertising.