As one whose whole collection can be bought if 
you are willing to meet my prices(and I mean 
really obscene prices too)
its completely ethical to sell for what the 
market will bear.
I, being a Tomoyo freak, have watched the prices 
for those puppies soar over the last few year, 
and I know what most of the Japanese sellers 
paid for them. 400 to 800 yen initially from the 
studios and then in the secondary market, 3000 
to 6000 yen.
I am willing to fork over 200 or 300 dollars for 
a nice cel of Tomoyo, that  I know was purchased 
for less.
So its what the idiot collector(thats me here!!)
is willing to cough up that matters.
Always remember, Caveat Emptor.