It all really boils down to, if you're not 
pleased with the cel, check the auction site.  

Did they describe the cel in detail, stating 
that the background/A layer was a copy?  If not 
I would complain, try to get a refund and send 
the item back.

If the auction did state the condition of the 
cel, and about the copied layer, it's kinda just 
one of those things you'll have to chalk up to 
tough luck, as much as you hate to do so. :(

Anyhoo... good luck in whatever you decide to do.

~yaten