And why are you defining hard-hitting journalism here as a reporter demanding that Cannon provide a receipt instead of a reporter demanding that WotC provide some evidence of wrongdoing?
WotC didn't and doesn't seem to be looking to punish, or otherwise penalize Cannon. They just wanted the cards back. Cannon doesn't have to have done anything wrong for them to send someone to ask for them back.
And, if we must - the videos are evidence of something that's ethically questionable.
Assume he's being honest - he really did want to by the previous set... $4000 worth if I recall correctly. When he found they were something else, he's in the position of a guy who, in buying a backpack at a yard sale, gets it home and finds someone's wallet, driver's license, and several hundred dollars in cash in there.
He has something that he shouldn't. The ethically correct thing would be to return it, rather than take the extra value and use it for himself (by making videos). So, he kind of failed that ethics test.