In last few seasons, in my server it is hard to sell players after season half. It's almost impossible to sell them if their average quality point ends with 0, 1, 5 or 6. In such case, I suggest to sell these players in early of the season. For players approaching next star, be careful and consider if you need to hold his skill points or simply sell them. Age is relatively less a problem now (but I don't have 30+ yr old players for some seasons)