I know you've made up your mind, but I checked 
out this guy's other auctions and found this cel:

http://page6.auctions.yahoo.co.jp/jp/auction/f777
1559 

Then I saw this one on anime ed, which is either 
the same one or one REALLY REALLY UNCANNILY 
close in sequence:

http://www.animeed.com/upload/RanmaNoD1.jpg

Which is marked as 'sold'.  If you're really 
burning for info you could e-mail them a link to 
his auction and ask if they could authenticate 
or at least if they sold it to this guy.

Just a lead :P  There's no telling how many 
times this could have changed hands, but if they 
sold it a couple of days ago to someone other 
than this guy...

*I* wouldn't necessarily want to spend that much 
time on it but someone might fancy themselves a 
better amateur detective than I! o_~