I think you should have made it explicity clear 
in the auction description that you were using 
another person's scan & that your cel did not 
include a sketch.  It would have prevented all 
this hassle for you.

Often times an auction seller will simply post 
the image of the cel and the title of the anime 
it came, nothing else.  Call it laziness.  But 
most bidders will assume that the cel DOES have a 
matching sketch.  And in 95% of the cases this is 
true.

This occurs more often on Yahoo! auctions than E-
Bay.  Regular E-Bay sellers are usually more 
communicative of what's included and not included 
in an auction up front, because they want to 
avoid the pitfalls that you are going through 
right now.

Did the buyer have a right to assume there was a 
sketch, based on the image you posted and the 
lack of information?  I think so.  But does the 
buyer have an absolute right to take action 
against you because there was no sketch?  I would 
say not, but...

You are partly at fault here, especially if you 
NEVER mentioned that the auction displayed a scan 
of a different cel, and not the one you were 
actually auctioning.  That is grounds for mis-
representation of the item.

If you were going to use someone else's cel image 
(which I hope you asked permission to use), you 
should have made it abundantly clear that it was 
NOT a scan of the actual cel you are auctioning 
but that your cel was identical.  Naturally, you 
should also describe the differences between your 
cel and the one in the image (the difference in 
sequence numbers, whether or not there was a 
sketch, etc.).  This would have been the smart 
and ethical way to go.

If I were you, I would extend my sincerest 
apologies and try to work out something with the 
buyer (a refund of $1-2?).

You are not obligated to do this and you may not 
be entirely at fault in this situation, but a 
fair seller should admit their mistakes and try 
to make up for them.  Most regular E-Bay sellers 
do and will.