There is plenty of information on this try reading some other threads .
Do you play Association games ? Then understand it is based on % ,so you as level 8 see a player at 99% so does a level 20 player
As for price that is obviously relevant to each level.
As for selling a player you are selling to the game...no person is bidding on your player.
Think about it as it does make sense it´s not rocket science