I belong to the school of thought that there's 
nothing wrong with retouching scans, just as 
long as you're not altering the overall image of 
the cel or imperfections (slight or 
otherwise)...especially when putting them up for 
sale.

Typically I'll do a lot of image editing before 
putting a scan up on my website.  This is for 
two reasons:  first, my scanner is small and the 
cel never lays flat, thus creating a shadowed 
gradient which shows up in the clear parts of 
the cel, and second, I have a terrible time with 
moire every time I scan a cel.  So the editing 
involves making a uniform white color for the 
background of the image, and then sometimes 
making the painted areas more uniform in color.  
Occasionally I'll have to play with the 
brightness or contrast features to get a more 
accurate color palette, since I am chronically 
plagued with that "washing-out" effect.

Of course, neither process is meant to improve 
the presentation of the image beyond its natural 
state.  On the contrary, all the editing is only 
done to provide a more accurate depiction (one 
that has not been subjected to the evils of 
scanning error).

That having been said, usually I'll rescan a cel 
and use the rough image on sales pages if I feel 
that there is any question at all about the real 
quality of the cel.  And I have never used image 
editing to fix any actual problems or damage.

Even the greatest-looking cels in person can 
look terrible if a shoddy scanning job was done 
(I know from experience!)...and since a large 
part of the value of online galleries is in the 
presentation, I really think some amount of 
editing is necessary and desirable.

But that's just my two cents.  (Or is it more 
like a bucky fifty by now?)  ^_~