i used to work for an autograph dealer. there are only 3 or 4 legitemate ones in the country. We did not deal is sports memorabilia accept the occasional purchase of entire collections.
I will tell you though that "the certificate of authencity" is really just the dealers opinion that it is a legitamate autograph. If they did not get it in person you are trusting their experience in that they have seen this autograph time and time again and knows the details that make it real. If you are dealing with a legit company I would trust this.
For a time I was able to spot a fake where my boss would just glance at it and chuck it. The autograph business gets a bad rap because of so many fakes on the market.
For example, if you see a britney spears on ebay for 9.99 it IS fake. She never signs so when you see a real one, it took some effort to get so your going to see a much higher price.
Anyway, the COA is an experts opinion, so make sure you do your research before you spend the bucks to get it authenticated.
If your looking to see what its worth go to
http://rrauction.com and do a search. they are very picky on what they put up for auction and i would say 98% of their stuff is good