Ok, perhaps it's a little difficult but try to 
follow : The "artist" sell the cel by clearly 
stating that it's not an original. The person who 
bought it then resell it without saying that it's 
a copy...  clear enough now ?

And yes, there is/was cels that are pretty 
expensive but that no dealers/collectors buy 
because there is too many copies from those shows.
If you want to sell a cel and the potential buyers 
already saw 4-5 copies copies around, they are 
much less likely to buy it even if you promise 
them that yours is the original and all the others 
are copies. Don't ask why : the reason is obvious
...

Cheers,
Yann Stettler