Folks ... this person was simply posting a warning that he believes that these cels are fakes. It sounds to me like he has some fairly good arguments to that effect (incorrect paint colors, etc). There is no "they're real until proven otherwise" in cel collecting, and we all know it. Otherwise, we wouldn't scrutinize images on YJ and eBay before bidding or buying.
If Guardians of Good wants to post here and defend himself, he is perfectly able, as he is also a member of this community. Let's not all jump down Mace's throat because he's suggesting someone here is of questionable character. It certainly wouldn't be the first time someone has accused someone else in the cel community of doing something rather unscrupulous.
If you've actually dealt with GoG, then certainly your opinion matters here, but for those who have not, simply attacking Mace isn't exactly going to help anything or anyone else. I only see one post from someone who actually seems to know GoG and has dealt with him. I myself have had conversations with GoG regarding Transformers cels and my husband's Voltron cels, and have had my doubts about him, but I'll leave it at that.
As for whether or not his GIJoe cels are real or fake .. I don't know. I know Guardians of Good has tried selling a cel of Optimus Prime in the past that was mislabeled -- it isn't/wasn't actually Optimus, but certainly looked a lot like him. In fact, the original buyer of this cel found out and GoG was forced to cancel the sale. He then re-listed the cel in the RS shop, where it still sits:
http://www.rubberslug.com/shop/item.aspx?i=3910
That cel is actually Ginrai, not Optimus, and yet even though he's listed Ginrai in the title of the cel, he's still trying to pass it off as Optimus Prime even now! God Ginrai is not Optimus Prime. Based on that alone, I would certainly have reason to doubt GoG's truthfulness about the cels he is selling.
Just my two yen.