I think its cuase people see them as a set, so 
in a sense, you are breaking up a set. (like a 
set of dishes, etc... or a figure set...)

since alot of people would PREFER to have 
the sketch AND the cel that match (and 
background if possible... I know I would...) 

by selling them separately, if someone else 
buys that cel later, they may never find the 
other matching peices, while if you had kept 
them together, they might have been able to 
have the set.

I think that's the gripe. now, mind, I'm playing 
the devils' advocate here, as personally, I think 
you should be able to sell your sketch if you 
want. especially if you dont particularly like it 
and someone else does. I was just trying to 
offer insight to why people dont like separating 
cels and sketches.

my friend steph can't afford a princess 
mononoke cel. she ADORES the movie. she 
was very very lucky in that curt let her buy a 
sketch from a cel/sketch set that otherwise 
she may have never been able to afford. so 
now at least she has a peice of artwork from 
the movie.

i think you can take it both ways, people 
should just be more understanding of different 
ways of feeling about sketches or cels.