Check this thread
But here is how you do it:
Step 1: Keep your free youth in your team.
Step 2: Train and assign skill points until he is exactly 1sp from the next star.
Step 3: Train him a special ability.
Step 4: Wait for the offer you couldn't resist.
If he gains skill points, spend them on training new position so that his quality won't increase.
He looks perfect for someone else's level 20 team at peak 5star limit with special ability, and is younger than players on the scout list.
This selling strategy is only possible above level 6, where you can sell 3* players this way.