most Saiyuuki cels are reproduction.  But there 
are indeed certain cels that Pierrot produced to 
be used in magazines.  You just need to be 
careful about who you're buying from.  Douga and 
genga had been available for sale for quite some 
time, so it's very easy for anyone to just sell 
the repro, add the genga or douga they found and 
call it a production.  But in your case, "that" 
seller is quite legit so I wouldn't worry too 
much.

People, you just need to be extra careful when 
buying cels.  Not long ago, another YJ seller was 
selling some reproduction cels, claiming them as 
production cels!!  So, please be very careful if 
you're bidding in hope that you're bidding a 
production product!!
Good luck!