Well, I don't consider myself a "bigshot" but I 
can give you MY reasons for suspecting fake 
cels...and of course, what puts the evil in 
evilbay; ahem eBay.  ^_~

Some cels just don't look right, be it the angle, 
color, shading, ect. There are ALOT of good 
artists out there that auction or sell what they 
claim to be authentic cels. In the end, the buyer 
gets screwed.

eBay is evil. I use it and know it's evilness. 
^_^ For one, they have foolish rules about what 
they consider "adult", the cause of many, many 
removed auctions that don't even contain sexual 
material. (one of my biggest grips with them)

Other evil-bay point: dead-beats get off fairly 
easily (leaving you with the bill)....
it goes on and on (anyone want to continue for 
me?)