I understand where you are going with the 
separate layer restoration, and why to you, a cel 
with line restoration on the actual cel declines 
in value. All very true! 

Here is the way I see it, if I buy a cel that is 
restored by others, sure, I won't pay the equal 
amount if the cel was left alone, but if I 
restore the cel myself, that cel become more 
valuable to me, simply because I've spent time 
and effort on it. For a cel to occupy my time and 
effort like this, it will more than likely be in 
my collection for a while. And there you have it, 
that's my I restore faded cels myself, other than 
that, I do it because after restoration they look 
much better.

Here is fun question for ya: say if I restored a 
cel for you, and I become a famous animator in 
the future, will that change your mind about the 
value of that restored cel(btw, that's the field 
i'm studying in, and pursuing)? 

-manslayer