That sounds like collusion to me. I believe in market forces, and have told owners many a time who complain about trades - even those that I feel are terribly unbalanced - that the market is at work and that they should have made more inquiries and/or better offers. Squeaky wheels get oil...or however that saying goes.
Collusion, though? Completely different story. The one and only rule I've ever had in my leagues is that I will not allow a deal if I think it's collusive. Even if a trade is balanced, I wouldn't let it go if I felt there were shenanigans behind it (say, two owners ganging up because of a mutual dislike for a third). That type of situation has never happened, but that's how I would handle it if it came up.
If you feel you have enough to substantiate a charge of collusion - and definitely if you have the league behind you - you have not only the power, but also the responsibility to strike it down. Not doing so could allow/foster similar deals in the future.