As someone mentioned above, it is best to mention 
that a cel has been restored.  In reality, one 
should always mention to a perspective buyer if 
the cel has had any restoration done to it.  To 
not tell the buyer would be flat out dishonest.

Like any other services, there are better 
restorers and so so ones.  If you had SR labs 
restore your cel for instance, people will expect 
that is was done 100% correctly and only the best 
care was given to the piece.

Some series people expect the cels to be restored 
from.  Some series are notorious for not being 
handled well or the majority of them just 
discarded (though this typically is true mostly 
of old American cels, some of the older Japanese 
studios practiced this as well). 

One important piece also, good cel restorers are 
real professionals.  Many worked in the animation 
studios and so on.  If there was a complete layer 
missing from the setup, and you (as the customer) 
wanted that added back into your cel setup (for 
framing or what not) the final product would be 
far from a fan cel.  An animator/artist would be 
the one to make the missing layer, someone who at 
the least would have held that job in the studio 
at some point in his career.