I hate playing in fantasy football leagues with naive owners. All it takes is one good salesman to sweet talk that owner into giving up a Pro Bowler for a fringe starter.
And what happens when one owner screws another in a trade is that it upsets the competitive balance of the league and puts everybody else at a disadvantage.
The way a trade works is that both teams give up something and get something different in return, but both teams arguably come out better than before.
If this formula is not met, and if one owner clearly is getting the shaft, there has to be a way to stop those kinds of shenanigans.
This is why I propose that your league allows trades to be vetoed by a simple majority. If more than half the league feels a trade sucks … it sucks.