Hi,
At the end of the season(now) it´s a bad moment to sell, because everybody is waiting for the draft, except if you don´t want to continue paying more salary to this player even if you know that the bids for him are going to be worst.
At the beggining of the next season, everybody is going to sell the worst players received from the draft and the market will be bussy.
I think that the best moment to sell players in his fair price it´s in the middle of the season, but as i told you before, only if you want to coninue paying salary until this moment. In other case, sell now.
Good luck