IMO it is simple, do not post the link to your
site. If you concerned do not let people know,
heck us RS and don't make your gallery public if
you are that concerned. Personally I don't want
people taking the cel images I scanned, and not
b/c of fancel fears. People usually do not collect
the same series as myself. 

In any event, regardless if the cel was originally
stolen IMO you do not own anything other than the
physical cel itself. You do not have legal rights
to the use of the image of the cel, since almost
no cel collectors have the legal license to the
characters for the series we collect. If you
possessed the license you can tell people you own
the rights to the use of the image and all artwork
for that series, including the characters
portrayed. Furthermore even if the cel was
originally stolen, how the hell will you the X
numbered owner even know? It is not like when you
purchase a cel, you get the ownership invoices.
Sheesh. So on that note, screw what the site owner
says, neither of you have a legal toe to stand on,
both of you do not have the right to claim
ownership. At best you the owner of the cel, own
that cel, but the right to its use. If the studio
or the creator(s) that have the current license
came after you, then they can tell you that you
must remove or return the item. But lets be honest
what are the odds of THAT happening? Just my two
cents, but as I said earlier if you are concerned,
do not post your gallery online it helps reduce bs
like what you encountered IMO.