I have to agree, they do LOOK fake. There's 
something about the colors and the way the lines 
are drawn that don't seem right to me.  I guess 
it could be an inexperienced artist, but I 
personally wouldn't buy the cels (lack too much 
detail for me.  ^_^)
 An easy way to tell if it's legit might be to 
examine the matching sketches.  Most CCS cels do 
come with the sketch, and I haven't seen a fancel 
artist that faked a corresponding sketch- not 
that it couldn't be done, but it seems unlikely.
The other thing I think is key when trying to 
determine if a cel is fake is colors.  The colors 
SHOULD match because each studio uses a lot of 
pre mixed colors.  Granted, certain scenes have 
different colors for times of day or lighting 
effects, but there are almost always other cels 
in the sequence that have those colors - laid 
side by side if you see colors well it's pretty 
obvious if they're the same or someone's attempt 
at a close match.  
my 2 cents.