My theory, for what it's worth, Roger, is that the 'artwork' was probably drawn long ago, shortly after this strip appeared in the paper, by an aspiring cartoonist and fan, then framed by his/her Mum/Dad. Decades later, to land in the hands of an eBay dealer, who assumed it was original, and estimated its value accordingly. I doubt if this was in any way a devious attempt to con anyone, and the lack of response to my message was probably down to embarrassment rather than guilt.
Were it to be more sinister, a lightbox would have been involved and the linework traced far more accurately, with much greater attention to the lettering. Like the fake Beau Peeps I've been selling for years.