On new season he'll be 3* going for 4*, meaning there'll be more appealing players to bid war for.
Also, each one more year the player loses about 20% of its previous season value, for which the gained star won't cover all. My advice is to sell him before season ends, or immediatly. Either way it's better to have him to 4* missing one skill point for the 5th one, simply because lv 2 will be able to see it aswell, and bid on it as well. The more can make offers, the more you'll gain.