Although I don't see anything wrong with getting 
a random background and placing it behind a cel, 
or mixing backgrounds in your collection, I do 
feel that the original background should be kept 
with the matching cel when selling, along with 
the matching sketch. Matching backgrounds can be 
pretty difficult to find, and once such a set is 
split, it will probably never be put together 
again (such a tragic tale)

TTYL

Karen