I have kept my season ticket on despite moving abroad as I still want to be able to go to a game when I'm at home. To sell my tickets the only portal I use and aware of (other than mates) is StubHub. I wish the club would take this role on, where we can hand back our seat to the club to sell to fans for face value. The season ticket holder gets the amount of their annual fee/19, with small added amounts for the different categories. The club then makes the 'admin fee' from the difference between the face value and the amount received by the season ticket holder.
I hate seeing fellow blues fleeced by other blues just because they are not in a position to have a season ticket or want to see a game.