Wouldn't that make it easier to tell that the cel is 
fake if they come with a fake sketch?  The type and 
age of paper like babeyj69 mentioned, the clappy 
colored job, the comparison to the cel, and else.. 
can make us easily spot a fake cel, right? Even 
though some seller might sacrified their fake cel with 
the real sketch, it is more likely for us to have more 
chance to compare if the cel is real or not.  And if I'm 
that seller, I would rather keep my real sketch in 
case I want to make  more fake one.