When the item is signed, it loses its 
originallity, therefore the Market value would be 
lower.  I feel having a cel signed only increases 
its sentimental value.  Like Cres said before in 
the Elvis Record Collection example when the lady 
had Elvis himself sign her record, there goes the 
pristine collection and its value.  Thats just 
the way way rare collectibles are.   Another 
example, a copy of Spider-Man #1 signed by Stan 
Lee cannot be graded in Mint Condition - cause 
its not original, so its value would plummet.  

Signatures on common items like sports equipment &
(Cap, Ball, Bat), Photos, Shitajiki etc, would 
have an increased market value.  But it is 
necessary to also enclose a Certificate of 
Authenticity to claim the signature is valid.