I am not sure about that seller. Although they have some authentic stuff on their site, they also have had some ttm and secretarial stuff on there. If I go on just a comparison, this autograph does look to be authentic, but the reputation of the seller worries me a little bit.
If I can direct you to another good legit seller, try this seller on ebay: gridironlegends 100% authentic stuff. And they are cheaper too. ;)
http://shop.ebay.com/merchant/gridironl ... &_osacat=0