Personally i wouldn't sell that Guitar man, and secondly i wouldn't buy it. BUYING something thats signed doesn't seem to have the same value as getting it signed yourself, espeically when the band is still kickin. I mean i WON a signed fate of norns cd, which is a bit different becasue it cost me 1 email with my name and address.
I have so much shit i'd want AA to sign so i can frame it all and have some massive Amon Amarth framed wall of all cds, lps, photos etc...One day it'll happen hah.