I've got a good collusion case in one of my leagues. This is the 1st year of this keeper league. The commish's team is 2-8, he's pretty much done. His wife is 8-2 and in 1st place. He traded her Victor Cruz, Julius Thomas, and his 8th next year for Martellus Bennett and a 3rd next year. Now keep in mind, we can only keep 2 players and they can't be from the 1st 2 rounds or FAs. So Thomas wasn't drafted so he isn't keeper eligible.
Still this trade tilts way in favor of the team getting Cruz and Thomas. The reason I think it's even fishier is I offered the commish a 1st for those same 2 players and he turned it down. Something definitely seems off.
I see where you're coming from, but I think the only basis for collusion is that a husband and wife traded with each other.
Let's break down the deal.
Wife gets
Victor Cruz - Big name, big first month of the season. Hasn't done much since then, at this point is a mid-level WR2 with potential to be better.
Julius Thomas - Great player this year. However, can't be kept, so is basically useless to husband.
8th round pick - Just okay, could go either way based on who it's used for.
Husband gets
Martellus Bennett - Currently TE9 in PPR. I'm presuming eligible to be kept. Without knowing the husband's roster, there's a chance Bennett is a potential keeper.
3rd round pick - Not as good as a 1st round pick next year in theory. But since you can't keep 1st and 2nd round picks, he gives himself a good chance at two top flight keepers for the year after. Easy to make the argument that in a keeper league, 3rd round is just as good as a 1st.
In my mind, the trade value alone passes the sniff test. You can say the wife clearly wins the trade, but I don't think it's completely lopsided as to completely upset the balance of the league.
So we go back to the issue that they are husband and wife. Let's say for argument's sake the deal you offered is better than the deal the husband took. Does that constitute collusion? Here, I'm not quite sure. He still took a deal that was beneficial to him, that was not absolutely lopsided.
I'm in a league with my fiance, but I help her with a lot of her moves, sometimes even logging in as her to make add/drops, set her lineup, etc. I made the decision from the start that we would never trade with each other. But that's just cause it's a friends league, and I want to avoid any potential controversy, since the money isn't enough to create the headache. But I'm not sure if it qualifies as collusion if both parties conduct a transaction where they both benefit, even if one benefits more than the other.