I feel the only right thing to do is to sell it 
as a set. If you love it that much, spend $1 at 
Kinko's and get a nice color copy of the sketch 
in question.

It really bugs me when ppl deliberately sell a 
cel without the sketch and don't fess up about 
it. Sometimes people sell them separately to get 
more $$, and in general I avoid buying cels from 
people who separate the artwork. It's like 
selling cel layers individually...you take away 
something important and special when you split 
up "sets". The grail for me is getting a cel, 
sketch, and matching BG because I adore the 
animation process and seeing the major steps 
right before my eyes. 

i have passed up some otherwise dream cels (that 
I was willing to spend outrageous sums on) simply 
due to the lack of sketch.

Ky77