If both teams are honestly trying to improve their roster, then no worries.
But you're offering your bench players to a team in hopes he will beat the team ahead of you. If he sends players back in a trade that aren't of relatively equal value because he knows this is just a trade to assist him in winning this week, then that is collusion.
Of course, if you don't say anything at all, then no one is the wiser.
Only 3 leagues this year. No sense in rooting for everyone in the NFL.