Unless the majority of your league members care first about the league's success over their own team's success, then you'll always have a problem when you allow owners to veto trades... which is 100% of the time in my experience.
My advice is to have 2 other mature owners serve as co-commissioners with the primary commish to make decisions. The commish has the final say, but the other two weigh in. That said, as a commissioner, I seldom veto a trade unless there is obvious collusion. If there's obvious stupidity, let it roll.
LFoD '08 [1st/3rd], '10 [1st/3rd], '11 [1st/2nd], '12 [1st/1st]
123Innings '08 [1st/1st], '09 [1st/3rd], '10 [1st/4th], '12 [4th/2nd]
BRAL '09 2nd, '10 1st
Cool 3.0 '08 1st