COA's mean nothing. Anyone can knock one up. I had a look at his Shane Warne signature and looked pretty good - but you never can tell. I suggest you try and randomly compare a bunch of the signatures and see what you make of them.
Regarding COA, i used to sell alot of autographed item's (jersey's etc) all my auto's i had personally got myself, buyers started asking me for COA, i told them im not a business, but had no problem making one up etc, which some people were fine about it, one guy wanted me to sign a "stat dec" which i did, at the end of the day do your homework if it looks suss then leave it..
i didnt check the ebay link, but if u wanna sleep well at night knowing u got something legit, i recommend u buy something thats been authenticated by one of the major companies (eg. Upperdeck Authenticated, JSA/PSA). im sure a lot of the autos may in fact be legit, but dude these days the coa can be more important than the auto itself .