Selling players is I think the one you haven't mentioned. Apart from that yes they are the only sources.
In my opinion it is a good setup.
If you want to win and climb up the leagues. My tip would be to drop down in your first season if you cannot beat the current competition and then fight relegation if you need to.. get a great player or two in the draft then come back and win the next season. Risky tactic but if you can't win now it is better than being third and not getting a good draft pick.