And to come back to cels, let me explain you how 
it often work when a dealer buy cels in Japan :

Often the seller will give you a whole bunch of 
cels from one anime. You go through them, pick 
many and assign individual prices in your head.
Then you tell the seller the total.
The seller will look at the cels you choose, 
assign prices and come-up with his own total.

Well, let me tell you one thing : if both people 
know their job, the result is usualy pretty 
similar. There is very little randomness in it.
Even if seller and buyer disagree, it's usualy 
because the whole scale of value for that anime is 
shifted up or down : Ie, all cels are considered 
to be worth more (or less) by the same ratio.
Or because there is an external factor. For 
example : You know and I know that those cels are 
too expensive but I paid them that and can't sell 
cheaper.

Cheers,
Yann Stettler