Well, it's not s simple to answer because it's a matter of strategy plus different conditions for each manager
Some factors for example
- What lv are you (in higher levels it's more difficult to find young 5* trainers)
- Is it easy for you to find fast trainers ? (in some servers is common, in others it's more rare)
- Do you have extra sources (greens to spend or token to buy another player), how much and how easily you can regenerate them
- What position has this player (a DC-MC-ST are easy to find, DC-DR, DC-DL,DC-DMC not so, AML-ML, AMR-MR are rare)
- Does he have any sa ?
- How is the roster of your team right now ? Do you miss that position or you thinking of it as investment for the future ?
- Did you use him in some games ? How did he play ?
That's very important.
Till then, if he is near 6*, add a sa or role to him so not get there and you cannot sell him in tm