In the strictest sense, anything written or 
added onto a collectable that wasn't meant to be 
there as part of the production is considered 
damage.
    If your 3 year old cousin scribbles his name 
across a cel in laundry marker its the same as 
if hikaru midorikawa does..in the strictest 
sense..they have both just damaged the cel.
  
How much the autographed cel is worth depends on 
who you sell the cel to.



^_^