It's also not uncommon to get a copied background 
included. A seller in posession of an original 
can make a copy, quality will vary. Not sure if 
an un-matching BG or a laser copy Bg adds to the 
value of a cel, probably depends on the 
collector's individual taste. Some people even 
collect assorted unknown backgrounds, they can be 
very nice and detailed.

I should probably add it's always nice when a 
seller notes if a background is a 
copy/matching/un-matching one.