Unless you have 4 better defenders next season, I would keep him and sell him toward the end of next season.
Sell on day 1 of the auction....If you sell now,you might not get to see a bidding war and that's not a good deal especially if you're in need of money
one disadvantage of waiting till next season is that the player's value will decrease about 10% since he will be a year older
i would train him a skill point or two more till he pops up on the negotiation list. then take the 1st decent cash and tokens offer that came along..