Yeah, but things get tricky.  When someone sends 
me a picture to reproduce for them, I generally 
don't know where it comes from, but other than 
that, I always list on my site where the cel's 
coming from 

(i.e. a 'reproduction'-from an existing picture 
(usually a screen capture), a 'customization'-
based off an existing picture, but 
w/characters/colors/scenery/etc changed or 
a 'original'-completely from one of my own 
drawings.)

Besides, I don't know about other cel collectors, 
but I can always tell my cels from the real ones, 
like the lines are a little thicker because 
they're hand drawn and the acetate sheets don't 
have those little holes at the top 

Like this one: 


Gets tricky though, doesn't it? ^_^