What kind of leagues do you play in that this stuff happens all the time?
Public leagues, leagues from browsing custom on yahoo while bored and drafting one February afternoon or another.
As I mentioned before, in my private leagues nothing gets vetoed. Everyone is trying to win. I'm just saying that in the many tainted leagues that I've run across in basketball, football, and baseball combined, usually the bad trades are because someone was either a beginner and really ignorant about the rules of the game, completely apathetic to his team, a friend who bribed or verbally coerced an ignorant/apathetic leaguemate who may have just been tired of hearing about it and thus clicked accept. They almost always get vetoed and they almost always don't create much of an aftershock.
People know when their trades are a little bit too lopsided. Most people are mature enough to let it go if they truly felt like they were getting away with one in the first place.
On the other hand, there are situations where public leagues or leagues with a lot of beginners are intimidated whenever they see big names in a trade and practically veto out of impulse. This is certainly a problem also. In leagues I've played in there have been trades that were initially vetoed, but then argued for by both parties honestly, and revoted on and processed the second time around. I don't have a problem listening to arguments/letting the arguments be heard from either side. Then the league votes on the appropriateness of the trade. This is their discretion.
Not to be too redundant, but I'll reiterate. In the serious leagues that I commission, there have not been any trade vetoes in three years and there have been 100 or more trades. When people are trying to win, it's not a problem you even have to think about.