Yeah I think nobody was actually "doubting" him in the sense that they were out and out SURE he did this with malicious intent, but his random spewing of "I'm in the State Police", "I'll get a Signed Afidavit that it's not tampered with by experts...", "I'm thinking of filing suit against those who warned bidders in my auction" remarks just had everyone basically looking at him as a jerk.
Not to mention, he's selling hotwheels on custom cardbacks as well I guess... That doesn't bode well for his credibility on resealing something and selling it... If I'm not mistaken he possibly denied even being capable of doing that?
Anyway, I think the point is more that he simplyd idn't WANT to take that down after being told it's a fake... Whether he did it himself or not, he was reluctant to take the auction down, didn't like the discussion on it, and lashed out. To me, he F'd that up himself and brought on the negative attention by replying like a tool instead of saying "I bought it and it' sealed so I just assumed it's an error".