The cel is DOUJIN-produced, which means that it 
was created by a fan.  (Hopefully, you now know 
what "doujin" means, which is 'fan', if you 
didn't know already.)

A representative from the Mandarake shop it was 
originally purchased from told me personally 
that the cel was indeed a doujin cel.

I have seen it up front personally, and it's 
still a very pretty cel (the fan service aside) 
and quite large.  The quality is as good as any 
hanken, and you never know... it may have been 
used as a hanken cel for some product (official 
or otherwise).

In any case, I see no reason which the cel can't 
be placed on sale again, so long as the 
important details are known.