Aside from what's already been mentioned, there 
is another piece that factors into the 
situation.  Take into account the person who is 
selling the cel.  Reputation factors heavily into 
the credibility of cels.  Any reputable US dealer 
(and sellers in Japan too) would not willingly 
and knowingly try to pass of a fake cel as the 
real deal.  It's simply not worth it to make a 
few bucks, where if they're caught they are 
instantly branded as a dishonest.  This hobby is 
one largely built on trust, and is something 
dealers aren't readily willing to sacrifice.