To get authentic sports memorabilia is a dream for all fans, both of a sport or of an athlete. Any item - ball, bat, jersey, cap, anything - that has been signed by the athlete becomes an item of sports memorabilia. The trouble is that signatures can be forged, and forged quite easily. To avoid getting scammed like that, one option is to choose reputable distributors of sports memorabilia to get your souvenirs.
The signature on the item only makes the item count as memorabilia if it has been certified by reputable distributors of sports memorabilia. Even if you get a certificate from the distributors guaranteeing its authenticity, unless the distributor has a solid reputation, you might be laughed out of the place if you ever try to sell or even show it off.
Many athletes and stars sign with particular distributors, often for staggering amounts of money. If you want an autographed item from your favorite star, it might be worth the effort to try and find out if that person has signed with any distributor, and, if he or she has, for how much.
People who claim to have gone to Michael Jordan's games and got a hundred signed shirts do not count as reputable distributors of sports memorabilia, no matter how convincing they sound. For one thing, while an athlete might be willing to sign something for a fan - they, by and large, respect their fans and are happy to do something like that - they will not be willing to sign a whole bundle of shirts like that, that is only common sense!
And if the price seems to be too good to be true, chances are that that is exactly what the deal is - too good to be true, and, therefore, not true. It is quite easy to fall for con artists that way, which is why it is always better to stick with reputable distributors of sports memorabilia.
Of course, if you are extremely lucky, you might just be able to meet your favorite athlete and get something autographed in person, in which case you will not have to worry about authenticity - and you will have met you favorite star, into the bargain!
However, if you are looking at the deal as an investment opportunity - which is quite a smart thing to do, nowadays - your word that you met the athlete in person will not carry much weight, so you will have to try and get it certified by the proper authorities. There are also websites where you can register your item. Once you have done that, you will have a solid investment in your hands.