I've heard figures of over 80% of eBay Autographs are fake. Since U2 is fairly easy to get, I would suggest just trying to get them yourself.
PLUS I heard Bono/Edge won't sign guitars anymore, so who knows if it was real?
I have autos from all except Edge.
(Might help to link to the auction, or the eBay # so people will know which one you are looking at too)