I don't know much about grandstand and their 'authenticator' other than they are not eBay approved nor industry recognized. I know I personally would never utilize them.
I am seeing their certs with bogus items all over eBay. Why would anyone use a complete unknown when they could use one of the top 3 industry recognized companies?
It almost seems like the company was made up in order to push fake items on people. That name is very familiar to me for some reason.