I worked in the industry of signed sports memorabilia for 5 years, be very cautious buying online by someone who isn't AFTAL (Autograph Fair Trade Association Ltd). A certificate of authenticity only means something if it's from a legitimate trader, otherwise any company can just print their own certificate to say they believe it's authentic.
If you are buying from a private individual, try to get some evidence of how they acquired the shirt and with a lot of signatures you can google what they should look like to compare yourself if you think it looks right.
Honestly it's a bit of a minefield and there are certainly a lot of bad actors out there so be careful!