It's always best to confront the seller first 
but...

Provide evidence of the damage.  Scan the bad 
parts and save the original image that seller 
displayed for you.  Compare and contrast the 
seller's description of the item (if there was 
any) to the actual reality.

Keep your dialogue civil and focused on what you 
can prove (that is, don't accuse the seller of 
anything but concentrate on the discrepancies) so 
that the seller will hopefully take your 
objective stance on the problem and agree to some 
responsibility for the cel's condition.

Of course, you probably will not be able to 
convince the seller on a full reimbursement but 
perhaps a partial one will be agreeable.  But if 
you're dealing with a foreign dealer, the chances 
are you may not make any progress at all.  Still, 
it never hurts to try.

AS FOR YOUR OTHER OPTIONS...

If dealing with the seller doesn't work out, I 
don't see any problem with restoring the cel, if 
the damage is that extensive.  Remember when 
Stephen remarked that he had to discard tons upon 
tons of cels because they were damaged enough 
(even if just a little) that he wouldn't think he 
could sell them at a profit?  If the resale value 
of the cel is not going to be anywhere what you 
paid for it, then I see no obstacle to you 
restoring it and making it beautiful again.

If you aren't completely sure about that, scan it 
and show it to everyone here.  You should get 
good input on what should be done with the cel.

As for selling the cel again, if you're not going 
to get anything worthwhile back to you, don't do 
it.  Sit on it for several years and (who knows?) 
cels will be so scarce that even damaged cels 
will increase in value in the eyes of future 
collectors.