You have to ask yourself why sell now and why sell tomorrow? Sell now you most likely sell your lower quality players which will result in higher average quality of your team giving you "better" draw for the cup. (Meaning you will meet better teams). Selling tomorrow you will get less money?
have you got a replacement? Player lose star and value but still shiny in the lower levels, lesser value will make up with bidding war. Most important of all you might need that player because pretty sure you won't find replacement so soon. That's just my thoughts.