Fast questions.

How do I unblock the other sponsors? I only have 1 option and I'm level 2 (if that matters).

Why I can't sell players? I keep trying to sell some players that I don't need, (but I don't need the space for another players for just fire them, I just don't need them and want to sell them) but I've never been able to sell any players.

Answer 1: Level 3 for "SPEED" and Level 4 for Topkers, don't know more.
Answer 2: Players just do not bid on your players, simple.

Try and sell them at a lower price.