You sure talk the talk with all that "knew or should have known" mumbo jumbo.
But seriously, I used to be a litigator, but I also have no clue. Regardless, and this isn't legal advice, if it were me, I'd let gon know that it allowed two people to be scammed through its site. Also, be careful og A'gon's "dispute resolution" service because it appears that you agree to indemnify them and hold them harmless. Obviously, a'gon didn't commit the fraud, but they let it happen. Twice. Not cool.