The only time I've had a similar situation is 
with dealers who don't have the time to update 
their sale pages that often.  Actually, I'm 
waiting for a dealer to take down a cel I bought 
a little while ago before I post the cel.

I, too, have poked into cel graveyards because it 
is interesting to see how someone's gallery has 
evolved.  However, in my opinion, posting a scan 
of a cel one no longer owns does potentially 
infringe upon those slippery 'cel owner rights' 
that we've struggled with in different guises 
before.  I am not particularly inclined to 
maintain a cel graveyard myself (not that I've 
sold any cels) but I would hope that I would be 
sensitive enough to a fellow collector's feelings 
to remove a scan of a cel that, after all, is no 
longer mine, if he or she asked me to.

Actually, please allow me to ammend that.  I 
would ask (respectfully, always!) if I could keep 
a small scan, in grey-tones, or maybe just a text 
description link to the new owner's page, because 
I like posting links to galleries that impress 
me.  I would mention quite clearly that I am 
willing to take the scan down if that is 
preferred.  If the fellow collector doesn't have 
an on-line gallery, or if the preference is to 
have the scan removed from my pages, I would 
acquiesce to his or her request right away.

For me, it is a respect issue.  One of the 
reasons some of us collect cels is because they 
are so rare and unique.  Having two scans of the 
same cel, even though the scans are different, 
robs a bit of that rarity and uniqueness, doesn't 
it?  To have the same cel 'appear' in two 
different places at the same time?  I can see how 
that could be upsetting to some collectors, 
especially for those cels that are quite 
expensive or very rare.