My observations lead me to believe that this cel 
and its seller are legitimate, although I will 
caution that it's just a gut feeling on my part.

[1] The seller has a good feedback rating, with 
no negatives.  Even the one neutral comment 
stated that the seller was very nice (although 
that might not have anything to do with the 
authenticity of the cel).

[2] The seller also auctions off non-cel items 
(and they seem to be authentic Japanese 
collectibles to me, although I'm no expert).  I 
tend to be believe rip-off artists would 
concentrate only on selling one type of item (in 
this case, cels).

[3] The fact that the seller has Buy-It-Now 
prices on all his/her auctions (and many equaling 
the starting bids) might indicate a desperate 
need for money quick.  I don't think a rip-off 
seller would settle for low Buy-It-Now prices 
(shouldn't they try to maximize their profit)?  
ALTHOUGH, it might just mean that the seller 
could want to sell "hot" goods quickly before 
they get caught.

[4] One of the seller's current auctions has a 
cel with the character's eyes closed; rip-off 
artists tend to sell the too-good-to-be-true 
items.

HOWEVER, I am concerned that the cel images do 
not display sequence numbers, nor did the seller 
provide any in any of his/her cel auction 
descriptions.  There is also the possibility that 
the seller honestly does not know whether or not 
your Belldandy cel was an original production cel 
or official reproduction cel.

So my final answer is NO, I don't think the 
seller is trying to rip you off, but I DON'T KNOW 
if the cel is an original production cel or an 
official reproduction cel.

Let's see if the cel experts here can shed more 
light.

*

PS - There was also a previous thread about this 
seller that you can read up on:

"Maybe I am being a little suspicious ...But!!" - 
youaretheone