Cel shops don't always know about the anime for 
which they're selling cels. They might have never 
seen it at all, they only know that characters X, 
Y, and Z are popular, and then they pick cels by 
picking ones they think are attractive or have a 
valuable quality of image.

I notice this since it's a big thorn in my side 
when it comes to some of my interests. Example: I 
love Miyu shinma, but they're (for the most part) 
unknown. Dealers sell mostly just Miyu and Larva, 
and the few other characters that everyone knows. 
Most dealers either haven't seen the show, or saw 
it once/partially and don't remember enough to 
recognize minor and/or episode specific characters
unless the character becomes very popular for some
reason. Example relating to Miyu: Despite being 
just as minor as the other shinma, most everyone 
knows Shinma Maji-kan (aka: Moru, aka: the 
Butterfly Shinma). She happened to be pretty and 
became popular, so dealers started finding cels 
of her pretty form. (Cels of her less pretty, 
shinma form are still nowhere to be found. . .)

Many Sharp Smiles,
--Drac