I have absolutely no qualms about breaking up a
set if I don't have the whole thing to start with.
 There have been several cases where I've had
most of a sequence (twice in sketches,
twice in cels), and eventually sold/traded them
off or gave them away individually.

On the other hand, if I happen to have the entire
package of something, I'd never be able to bring
myself to part it out.  It would either stay with
me, or be sold/traded as a set.

Why?  Because it just seems right to me that way.
 *grin*  Your mileage may vary, and I wouldn't
thwack somebody else for doing it, but I know I
personally wouldn't be able to.