Yeah, i would say that this is a real cel.
I can't tell any differences from the sketch and
it looks like something I've seen from the show.
I can't place the exact scene though.  I've 
purchased from this person before (a nice 
CCS cel).  Our transaction went well.  
So I wouldn't worry about the auction.
I'm sure that it is a real cel.

Faris Kalin, bard extraordinaire