I saw this online however I do not know if it's genuine. The person selling says they got it at Comic con a few years ago and there is a certificate of authenticity from Shadows on the Wall however I know he did not generally sign any Potter stuff.
Definitely not Rickman. Alan stopped signing Potter material after the release of the first film. In short order, he realized he would never be capable of living his life if he did sign Potter memorabilia. He knew he would never be able to go out in public or get thru a crowd without hundreds of fans swarming him. He did on occasion sign for small children or charity auctions, and I assume for friends and coworkers, but not the public.
After a Broadway performance someone asked him if he would ever sign Potter stuff, and he responded..."maybe when the hype dies down," unfortunately he passed before the hype did.
I am sure he would sign books and the like for children at the premieres that he attended, at least for the press cameras, but more often he would walk the red carpet and wave to the crowds while doing minimal signing.
He most certainly did not sign the piece that this thread is about.