About selling "special" players and attachment to players... Im only in my 4th season but Im afraid that Ill have to sell two of my old favourites this or at the beginning of next season:

Taj Broadbelt: my generated "star player" from season one, an one-on-one goalkeeper, has turned 22 this season and his training progress has dropped noticeably... Im afraid Ill have to get a new goalkeeper next season since its unlikely he will hit 5 stars again this season.

Jonathon Jackson: "Action Jackson" My AMC/AMR which I bought early in my 2nd season, my first big buy (5 stars then I think, and grew to 5 stars again in 3rd season)... he was my free kick taker and best scorer in 2nd season, and still good for some goals and assists in 3rd season. Now he is barely 4 stars and the weakest player in my top11. Ill have to replace him sooner or later, but Ill miss him.

My current favourite is the first scout I bought (beginning of 3rd season):
Rene Leitner: Striker with free kick SA. He scored 52 goals, about 15 assists and 16 MoM in his first season. Now he has turned 23 and is back to 6 stars, but in the 2nd cup game he showed whats still in him: he scored the first 2 goals and one assist, and MoM of course. Im sure he can keep going for a few more seasons. For somebody who only farms tokens but doesnt buy, a scout lasts longer because he will still be ok at 4 stars.