Hi ^_^

 First let me say Welcome to this very EXPENSIVE 
and ADDICTIVE hobby!

 I will try and give you both the good side and 
the bad side of purchasing signed cels.

 Some people would agree that a signature on a 
cel would increase the value thus thinking that 
the voice actor, director, animator, etc would 
not sign a cel that was not from the original 
anime. The idea being that the signature can 
actually authenticate (sp) the cel. I know many 
fellow collectors that have either purchased 
signed cels or actually gotten their cels signed 
in person. The signature (to some) can bring more 
value because more of the show has been added to 
the cel ie, director's, animator's voice actor's 
signature etc. The cel becomes more intimate and 
more desirable.

 However, some people would agree that signatures 
on cels actually decrease the vaule. First let me 
say that in my history of getting items signed 
the actor, director, animator, etc doesn't care 
what they sign as long as they sign their name, 
sorry but that is reality, try having hundreds of 
items shoved in your face by fans...you think 
that they are actually going to review the items 
that people want them to sign?....Think again. I 
personally would never buy a cel that has been 
signed. For one, you never know if the signature 
is real and if your like me, I like to purchase 
cels for owning part of the anime and I really 
don't want a bunch of gibberish on it. I know 
several collectors that unfortunately have had to 
pass on their Dream Cels because the owner of the 
cel/sketch had been signed...The signature 
carried on over into the image and in one case 
the signature was right on top of the 
image...sigh...idiots!...this is where the 
signature is bad. ....Hey look, it looks like 
Misa Hayse's arm....but what is all that crap on 
top of her?....Oh yay, I have a signed Macross 
Misa Hayase's arm because someone decided to sign 
over a full frontal, open eye shot, of Misa 
Hayase standing at her control panel....Don't 
believe me... I have seen it...

 I know this is long so to sum it up I would say 
the majority of time the increase in value due to 
signatures is purely personal.