What exactly do you consider collusion?
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It's going to be long, but here's my 2 cents.First I think it's worth saying that the dictionary definition of collusion doesn't seem as broad as many people use it in an FF sense. And that always seems to get pulled out by someone as an argument in these threads, which I don't buy or care about. There are a class of collaborative acts which I think go against the spirit of FF leagues. Some exactly fit the dictionary definition of collusion, others don't. I don't know of a more appropriate word for the ones that don't. But whether they fit or not, I think unless a league all agree that they are allowed, that most leagues would wish to not allow them. Ok, so let's jump into them.
1) A transaction or series of transactions made with the owner's belief they did not improve his team. For it to fit the dictionary def. of collusion both teams would have to be in on it. But in a practical "what should we stop in FF" sense, if Marc Levin gives me good players to help my team at his team's expense... even if I wasn't aware Marc felt he was hurting his team, it's collusion to me. The only difference is, if I wasn't aware of it, Marc is the only one guilty of collusion. If I was aware of it, then we're both guilty of it.
Any time something outside the "normal" FF assets are part of a deal, it would be collusion unless the league expressly allowed it. Like selling LT to a team for Chris Perry and $50 cash would be collusion, as the $50 would not be recognized as an asset that improved the FF team giving up LT.
2) Teams that act cooperatively beyond the level normally expected in FF. This one can be shades of grey. "You draft a good backup kicker, I'll draft a good backup TE, we'll both make sure our TE and kickers and backups don't have conflicting byes, and then we'll agree in advance to swap our backup players so we both get our bye's covered by above-average bye fill ins." I think most people will agree those teams are acting in concert beyond the level that a game of individual competition allows. This tactic is also frequently called sharing roster space, but it's also wrong because they agreed in advance they would do this and adopted joint strategies as a result.
Another example is saying, "MT doesn't have any RBs and it's the 4th round and he doesn't pick until the end of the 5th. Let's everyone draft all the RBs worth having before he picks again." This is one of those grey areas. Using the wording above, an owner is lobbying other owners to work collectively in a manner that goes beyond individual competition. But I wouldn't object to an owner saying aloud, "Damn MT, I would never take on the risk of not having a RB before my 5th round pick," but leaving out any suggestion or lobbying of actions other teams should take. Though some might say pointing the fact out could be poor sportsmanship, and make a good case for it, but I wouldn't call it illegal anyway.
Still another would be teams coming to prior agreements, whether it be to trade a set of players, then later trade them back... or to both agree they'll do a trade on week X. It is one thing to sound out other teams and find out if they think they'll be amenable to a trade at a later date. But to commit to it seems to go beyond the realm of individual competition and transactions that execute immediately.
3) Sharing roster space. Already mentioned the example above, if they swap players, then swap back later. Another example is the two-stage trade. "I'll give you LT and Rod Smith for Holt and C. Taylor. But, I'll only do if it we swap LT for Taylor this week... and then 2 weeks from now swap Rod Smith for Holt." What that amounts to is one team loaning LT to the other team for less than his value until the second half of the trade is executed. Again, I think most people would not expect that to be part of a game built around individual competition. The fact we can make a swap of players in a trade doesn't mean we can loan them out for extended periods of time.
Given all that I've said, I don't think the original trade posted is a problem as long as the two teams did not do either of:
1) Agree in advance to have one get a bye filler at one position while the other got the filler at the other.
2) Agree to trade back the bye week players once they are done.
If all they are doing is to pick up a player who will start for them, and they didn't give up a more valuable player to do it, then there is a reasonable expectation both teams have improved their gametime efforts.