You make a good point, you cant tell other coaches what and how to train and you normally would expect other coaches to train players in a certain way.
But this is where I think is the main issue in this game and why I love this game. In hattrick, i hope you all played this game, a midfielder you train by training playmaking to a high level and than train a little bit passing. in buzzerbeater you have several different options on how to play a player (this is my opion so dont tell me I am wrong) and there are thousands off opions when you ask thousands off coaches... If you read the forums people never agree. However, instead off trying to understand why someone has an opinion or trying to explain why they have made that decision on how to train their players, they keep telling the others are wrong or repeat what others already have said.
However, you can "educate" players. (this is the wrong word because it sounds patronising, but dont know what other word to use). Let me try and explain. As I said I have seen many good potentially U21 players being waisted not being trained and being sold for too high prices. As I agree, you cant tell a player what to do, however I can ask a player to train the player and if he informs me he isnt going to, ask him to sell him if he isnt already doing so. If he than asks way too much, I would advise him to lower the prize to something I feel is more accurate and will guarantee him a sale. In this way, everyone could benefit from this and I would also address this. The team in question does sell the player and get extra income, and doesnt have to pay the players salary. Also he might sell him at the start off the season, instead off realising until later in the season or the start off the next season that he needs to lower the price. Off course in that case it is too late for himself, because the player will have less value and valuable training time is being lost.
I dont think that there is much more you can do besides setting up a good scouting plan (I am quite sure there is a database off some kind already, since I did help scout in the past) and my idea above to advice and request players to be trained or sold in time so he can be trained... It is that up to the coach to train him and for me to fit him into the U21 team...