The problem occurs when one owner rips off another in such a lopsided manner that it changes the balance of power in the league. Then you've got 8 guys playing out the string because one owner is stupid and another owner ripped him off.
Again, what is ripping someone off? Does the guy giving up more have more on his bench to replace what he gave up? How do you gauge valuie in a 10 team mixed league? Everybody is loaded, so it makes it easier to appear to get swindled.
I sent an offer the other day of Mark Teixiera and Scott Kazmir for Carl Crawford. My firends think I'm nuts, but I have a loaded bench right now with Frank Thomas and David Ortiz able to play first, so Teixiera is expendable.
If someone were to veto that deal, I'd be pretty upset.
If you are going to veto trades, why not just veto draft picks? If you're worried about the balance shifting, why not overrule a draft pick like taking Juan Pierre in the second round?
These leagues aren't set up to be balanced. There will always be some stupid owner, and part of the game is to find that guy and get the best of him.
If you have someone in your league who gets ripped off easily, the answer to me is to not invite them back rather than havining everybody babysit people. Especially owners with something to gain.