First off, is this a redraft, keeper or dynasty league???
Trades shouldn't be vetoed unless there is collusion or cheating or roster dumping. Not up to other owners to decided what is fair, if they're all grownups let them manage their teams as they see fit. The concept of fair is too subjective, different managers rank players differently and people's teams have different needs. If someone is making stupid trades the solution is to not invite him back next year as guys like that always cause a lotta friction in leagues.
For more info on trade vetoes see: viewtopic.php?t=524568