...in your second paragraph, you're basically 
saying "Once I own the cel, no screencaps with 
the cel in it are to be reproduced", thereby 
claiming the ownership of the screencap which is 
absolutely absurd.
 All you own is a painted piece of plastic and 
the rights on the digital photograph/scan of it. 
NOT under any circumstances the production  
fragment the cel was used in, neither ANYTHING of 
the cel that was created from it before you 
purchased it ("you" applies to anyone, not only 
DotSē).
In the end, cels are production trash we paying 
collectors pick up. 
Bones are also unique. If you go to the 
slaughterhouse and collect the garbage/bones 
there, nobody will stop you, but you can't say 
you don't want the cows the bones were from to be 
cloned... they were not your cows. Their remains 
HAPPEN TO BE. Not fated to be.

Basically, I can go to AnimeCubed or any other 
Anime gallery, take one of those artbook scans, 
screenshot or whatever, and paint it for my 
personal pleasure, or even upon commission. If 
anyone happens to own the cel used to create the 
finished thing, well, so? The screencaps are 
accessible/available (within said grey zone) to 
the whole wide world without any cel owner having 
a bidding.

Also, you say, it's not ok to reproduce cel-made 
shots. Excuse me, if my wishlist cel never turns 
up, shall I be an idiot and keep on DREAMING 
forever as do idiots!? Hurray, it's so great to 
have dreams! Yeah, right..! Unfulfilled, what use 
are they? I will have my dream fulfilled by 
painting or having painted that one myself 
instead of going all empty-handed. And if I use a 
screencap, why should, or how COULD cel owners 
care?

That's the childish twist about cel 
collecting: "IT'S MIIIIIIIINE, nobody else is to 
have it!!" I personally couldn't care less is 
asked nicely. If it was only for selling 
purposes, ofcourse, I'd mind, but if it's sold 
later on because the painter no longer likes it 
or needs money or so... why care? As long as I am 
acknowledged as the original's owner and the 
fancel to be sold as such...

Quoting El Souzou: "Mrrreeuw! Rrrmau! Rrrr!" and 
scratching head with foot,
Ore-sama