Don't buy your autographs off a website. There is no fun in that. Go to a convention and have the actor sign a photo in front of you. You will know for sure that the autograph is authentic and you will have a wonderful memory of meeting him/her.
Not to mention you'll have a great time at the Con talking with other fans!
I have too many autographs, but to me that's less important than the actual meeting. I've had many friends take off to a convention or two during the year without me and ask if I want anything, and I never do. The autograph is more the proof of experience than anything.