With the way the system works, I tend to sell players that I know need replacing on the last day or second to last day of the season before the value decrease/age increase. There is still plenty of people who will buy.

Buying scouts on the first day of the season is advisable, because for that season they will be scout, and if they are a high end scout they can sometime be 1 quality away from being a star higher. So that's potentially 3-4 seasons before they will drop down to a 5 star player, if they are played and trained regularly & brought at a decent age.