I don't just mean that if you can buy one it takes the fun out of it. Personally I feel if the person didn't sign it just for you it means nothing. But also, I have heard stories that money hungry people who aren't really fans of a star or band will rush the person to get autographs to sell on ebay, often pushing away real fans who would have cherished them. And last, you cannot always be assured of the authenticity, even with a certificate. Who makes those things anyway?