The whole collectible market and current values, 
for any collectible at all (not just cels), is 
based on supply/demand.  If everything was based 
on the price it was originally, then cels would 
be free, old coins would still be worth a penny 
and the Barbie your mom bought when she was a 
kid would still only sell for a few dollars.  
Theres nothing unethical at all about selling a 
collectible for the going rate.  :)