More than that, I think it's better to think of a player as having a lifecycle on your club. The way I've always done it is generally when I buy players it's so that they can start for me, and then as my team needs change and trainees progress, etc., they transition into a backup role over the seasons. At some point when they slip further and further down the bench, it becomes time to sell them. Moving guys before they hit 33 is good too, but some players may not be able to keep up with your team's progress long enough to reach that age with your team.