I find no problem with it unless you're selling 
it...or if someone makes an offer and you 
entertain it.  Then someone should mention it or 
rescan it for them.  Especially when it comes to 
colors.

For example, I have a pair of cels that I bought 
mainly for the colors before I even got the 
anime...mind you, they're still awesome cels, but 
part of the reason I bought them was the color.  
As it turns out, the colors in the scan on the 
site as much brighter than the powdery shade they 
turned out to be.  I don't mind that much, but I 
would have liked the heads up.

Otherwise...I don't see the problem if it's in 
case of a cel where the image came out grainy or 
darker than needed.  In the case of the former, 
I've touched up the paint, the latter I've just 
brightened.

In other words...unless there's a trasaction 
going to occur, I don't mind. :-)

-Pitten