I've seen an "up/down" vote system in play before where a certain number of people had to approve a trade in order for it to go through. Unfortunately, it doesn't work too well. In my experience, people can get petty about people who manage to dominate or else remain a top team for multiple seasons that people collude in order to disallow all trades with the 'champion' player until he/she gets fed up and quits. It didn't matter if the trade was fair or somewhat lopsided in any direction, it would always be declined.