This may not help, heck you may already have it as a policy, but one possibility is to say on any trade you are even borderline uncomfortable about, you will notify all league members (league message board or email), and if 50% even or higher among those who vote in a time window (1-2 days, depending on what you think fits the league's level of daily activity; longer if it is not a daily transaction league) vote against the trade, then you will cancel it. Trade participants may not vote. *You* get to vote. You get no extra weight in the voting, but if your time window is fair, and no one else bothers to vote, by definition you cancel the trade (1 vote to none).
If this is not the answer, consider possibility #2: the league isn't worth staying in. Leave it, and find another... or start another, private, and only allow folks in you know as decent managers/owners from other leagues or from posts here or elsewhere.
amore cecidi tamquam blatta in peluim