There are many things you can say:
1. First and foremost, you should say that the point of trades is to improve your chances of winning. If both teams can make a reasonable case that they are doing that, then a veto should never apply.
2. You should say that trades are not always perfectly even. If one team is desperate for saves, he may trade someone of more value (i.e. Bonderman) on the open market than what he is getting back.
3. You should tell guys that are mad about a trade because they would have given more that they should have been making offers themselves to find out what is available.
4. You should tell everyone to decide what you want your veto rules to be: majority vote or commissioner approval. Sounds like they'd prefer the vote, but if they do then it sounds like no trades would ever go through. My suggestion is to make it commissioner approval, but only if 3 or more people protest. If 3 do protest, then get arguments/rationale from both guys involved in the trade. After that, if people want to take a vote then go for it.
5. You should get everyone to agree on what the veto philosophy should be: collusion only or anything that is "too unbalanced"
5b. If the philosophy is based on "imbalance", then get everyone to agree on what "unbalanced" means and how to define it.
"The government cannot give to anyone anything that it does not first take from someone else"