Because someone, at one time, may try to sell 
the
forgery to an unsuspecting person as if it was 
an 
original ? ......only if the cel is not 
clearly marked on itself (i.e. signature, 
watermark) from the artist.  However, there is 
nothing stated in the above-mentioned auction 
(or clearly marked on the cel itself from my 
perspective) that there is any sort of 
signature/marking by the fancel artist.  If that 
is the case, then it COULD potentially be REsold 
by the winner of that auction under the guise of 
a 'real' production cel.

And to take it further.......

If it IS resold as a 'real' cel, and is found 
out to be a fake, then it DOES make it more 
difficult to sell the original.  I know I would 
be wary of any cel that had tons of good fakes 
of it out there....