From now on, it's best if you direct Yahoo! Japan 
inquiries to this forum:

http://pub41.ezboard.com/fanimecelsforumfrm3

They tend to concentrate more on Yahoo! Japan and 
will probably give you better feedback.  Also, 
the Animanga board tends to discourage auction 
links provided by anyone besides the actual 
seller.

Yeah, I was looking at that cel a few days ago 
myself.  (Wasn't this a great movie?  So Ghibli-
esque but sad...)  Judging only from the scan and 
the description, I would say that this cel is a 
production cel.  Although I can't say that I'm an 
expert.  The seller states that the background is 
a color copy, and there appears to be sequence 
numbers on the corner.  Studio Pierrot 
reproduction cels usually don't bother with those.

Among the deputy services, Toru 
Sato is the most helpful when you want to ask 
the seller specific questions.  You could request 
Toru to ask where/how the seller originally 
acquired the cel, production sketch, sequence 
number, etc.