I'm more on the end of 'who can I sell to that is 
trustworthy' ^_^

But I've bagged a few scammers as well, and it's 
usually because although colors can be off, and 
my cel scanned on my computer is going to look 
different than your cel scanned on your computer 
and neither of them are going to have the same 
colors as the screencapture of the cel, the lines 
will usually give it away.

First, you've got your crappy lines.  The ones 
done with an unsteady hand that make any copy 
look obviously fake.  Then, you've got your good 
lines done with the wrong kind of pen, which 
makes your outlines either way too thick, or too 
light, or dark at the edges and light in the 
middle.  Sometimes these are hard to pick up on 
from a scan if it isn't great quality, so asking 
for a bigger higher res scan is a good thing if 
you're not sure.  Lastly, you've got your perfect 
lines.  Smooth, beautiful, and look like they've 
just come out of the press.  Because they have, 
or because fancel lines don't fade nearly as 
easily as real cel lines, if at all.  Cel lines 
are usually photocopied, and never quite as 
perfect, maybe getting rougher towards the tips 
of the hair or something like that.  And getting 
the lines in the exact place is really tough 
too.  I don't think many series (except maybe an 
old perfect condition prime shot snow white cel) 
would bring about the kind of money necessary to 
buy a special photocopier for cels, so 
generally... the lines have it.

...and beware of AMG cels because there's plenty 
of those out there that are both 'real' 
and 'fake' at the same time.