Go to roster click on player go down to sell this player, enter amount you want to sell him for, go down and click sell again, then wait to see if anyone bids.
Are you setting your lineups before you play a game? I noticed that your MVP potential and superstar potential players haven't played a single league game yet and I did not see them play when we played each other a few days ago. look to the right and you should see your next game where you can set your starting lineup.