There ARE collectors in Japan who have virtually 
monopolized certain series/scenes/characters.  
I've heard, for example, that there are a few 
Tenchi collectors who have hundreds upon hundreds 
of binders which represent a good chunk of the 
series.  You can virtually name a moment in the 
show, and they can produce cels from that moment.

Can I prove this?  No.  But it seems possible.  We 
have fanatics over here who would do the same 
thing if they could go to the corner store and buy 
cels for pocket change (which you could with 
Tenchi cels when they first came out).

But I don't think this is the primary reason for 
the cels you are seeing.  The answer is much 
simpler.

When studios in Japan release cels to wholesale 
and retail outlets, they don't release an even 
sampling from the whole show.  They release scenes 
and bits and pieces.  So that a specific scene 
will filter down through many dealers at once.

For example, I have a Tenchi character that only 
appeared in a certain outfit once.  And when I saw 
cels of her in that outfit, they appeared at a 
variety of Japanese dealer sites all at the same 
time.

Hope this helps.

JP