Maybe what the seller wants is to prevent the 
image being downloaded masively by people and fan 
cels done or similar based on that image.

Have never placed a bid with mandarake but maybe 
they have some kind of restrictions to allow bids 
like yahoo Japan has (having a credit card 
registered for example). This way they will only 
e-mail a high quality photo to a "valid" bidder 
hoping that if he/she decides to cancel the bid 
the image will be deleted and not distributed. 
Something like that. Or if they don't have 
restrictions maybe they will make sure that you 
are really interested on the item before sending 
you the image.

With that said, well, i'd like to add that I am a 
Rumiko Takahashi fan, and specially for her work 
on Maison Ikkoku, but seems that to be considered 
a "true" fan it's also required to have a 
laaaarge wallet. Of course there's no need for 
Mandarake to say that for sure we will like the 
item, it must be awesome without doubt, the 
question is if it's worth the amount they ask.....