I'm trying to understand the problem you seek to solve.
If it's a matter of someone selling a card to someone else at a discount, and they aren't violating the TOS, then it's not a problem in need of solving.
If it's a matter of someone's feelings being hurt because they aren't part of that deal, too bad for them.
I should be able to sell any of my unlocked cards to whomever I choose, at any price over the minimum that I want, as long as I don't violate the TOS.
The team already monitors (I will take them at their word) for TOS violations. If they catch people, those people should be punished in accordance with the TOS.
If I offer you my dupe Chicken Wolf card for 50k, and you agree to that price, and we both get on the AH and I list it and you immediately grab it, no one has been hurt. Sure, I could have sold that card for a few Kpp more, but I chose not to.
There is an entire forum section devoted to people making deals.