value will drop about 10% when he turns 19. Some are more, some are less. I would train him to 1 skill point from the next star so he can be placed on the transfer list and then leave him there. Or sell him via auction on day 1 of the new season during the afternoon/evening (your time).

Here are picts of my squad on level 5 and the next day when they became level 6 so you can see the change in player values.

players depreciation - best time to sell-contract-details-day-28.jpgplayers depreciation - best time to sell-contract-details-day-1.jpg