I agree with you. Don't sell him! Its better in the long run to keep him for multiple reasons:
1. It will boost the crap out of your merchandise
Some people have said that if you keep the good trainee you draft and he happens to make the U21 national team (Which yours has a decent chance at making) their merchandise has raised over 100k per week. Even if he doesn't make it, the fans still like to see the once rookie become the team leader and that also boost merchandise (As well as if he helps you win more games)
2. Although it may appeal to some people to sell your prized trainee. That could be the only good one you get in 4 or 5 seasons (Knowing these drafts), plus you hardly ever see a 5 potential guy with that much skill, so it would be a big loss.
3. It is simply more fun to train your talented stud. It's fun to watch them grow. I haven't been playing Buzzerbeater for that long but my one trainee that I have had, ive seen him pop 8 times (He is 22 now) and i get filled with excitement every time i see that he has grown as a player. Your especially lucky because your player has so much potential.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs