Heh, someone probably replied by now...I've had this sitting on the desktop for over an hour.
There are different ways you can let the trade process work. You can either have the commissioner look at them and make sure they are fair, or you can have the league look at them and have a "veto" rule if they feel it's unfair.
The veto thing is a sticky point in fantasy football because one person's view of a "fair" trade might not be the same as someone else. There's always the underlying thought that a particular team will now be better than their own team, therefore they don't want the trade to go through. That shouldn't be the point of the veto...really it should only be used unless the trade is blatantly unfair (meaning some pretty useless player for a top player) or if there is collusion between two managers to upset the league balance.
On Yahoo leagues, I think you have to have 1/3 of the members of the league voting to veto a trade before it's disallowed. That isn't bad if you're going to do it.
Basically, trades are fine as long as it's not dirty. If someone stupidly trades someone for another player without collusion or whatever, it's probably OK to go through.