I havent experienced that yet since last season I could sell my players only 3 days before season end

you dont always choose the time when you want to sell your player, you only sell your useless players after you know where you stand in all 3 competitions

for me I always have a full squad so I can be able to rotate enough during games and could reach as far as possible in cup and CL

so you can't just sell players in the middle of season unless you are sure he can never be useful again