Kinda reminds me of the old joke where the obvious murderer is on the stand at his trial and he says "You know who really did it? This prosecutor. Think about it. Why is he so focused on making me the murderer? It's because HE did it and he's trying to deflect the blame!"
Great Kids in the Hall skit.
Anyways, actual proof is very difficult to come by. Especially for autographs. To prove the auto a fake one has to verify where the auto was taken, if the person who wrote the autograph was there and where the person who received the auto was at during that time. There is typically too much to uncover to get complete facts in.
Most likely what will happen is that you will raise doubt to his auctions. Buyers may request information from the seller to question what you're telling them. Ebay will most likely do nothing. Authorities may not get involved in libel (can't be slander because it's all written) because the "cost of damage" is too small. Most you may get for that is a cease and desist letter -- which won't come formally from a lawyer because retention fees are too much for issues so small. Maybe the seller will block you from his auctions so you can't see them anymore? Not even sure if that is possible.
In the end, looks to be a whole lot of annoyances.
If you get any physical proof like mentioned above, then send it to the team/player/group that represents the autograph signer.