As far as I observe by checking my players, the selling price through "sell now" is 60% market value. This is reasonable and even a little bit more. I notice from other forum members that this can be even less.
Now rich manager has a new option to play. They can buy some players and use them as standby players for 7 days. During the 7 days, if any of his player is injured, they can sell him and immediately sign standby player to replace him. This may be an alternate way to save red or green packs.