after this message "You will receive 79% of the sale price after fees." should be a link with more info....in there will say (you sold #number# players the last 14 weeks , and that your player has been to team only for #number# days)
when you sell players this percentage drops...but as long as you keep this player to your team...this percentage will rise eventually...
so for example next season this player could be sold at 90-91% perhaps