I personally have no problem breaking the sets 
up.
I have bought a round dozen Chobits Douga sets
and a nice set of Genga/Douga from Mahoromatic,
as well as the Tiny Snow Fairy Sugar Douga I 
just got.
They all have a few real nice ones, and a load 
of not so nice.
I really have no need for a bank shot with 15 
eyes and mouths.
I take what I want and then offer whats left at 
a reasonable price to others collectors(mostly 
friends).
I "never" claim it is a compete set unless it 
is, I just treat it the same way I do when I buy 
a block of cels for one of them.
Sell the rest and try to make my money back.
I think of the cuts the same as when I got whole 
sequences of cels.
I only wanted a few, and there is really no more 
worth to a sequence of cels to most people 
except a few collectors.
As long as you dont try to pass it off as 
a "Uncut" cut, there is no problem.
"I step down off my soapbox..."