Hey, hi patrida.
Well, those players must be over 30 y.o. and the game gives them to you in the beginning of the season because your team hasn't enough players.
Yes, you cannot sell them because they are old and they are retiring, just sack them/fire them when you buy other players to have a squad of 15 players.
*remember that this is happening only in the beginning of the season. If you go out of players in the middle of the season (cards-injuries) , then you cannot play the match and you 'll loose 3-0.