Before bidding at any autograph auction, not just on eBay, even from a very reputable dealer, be sure you know that there are some reputable dealers whose guarantee of authenticity expires after a certain length of time, sometimes shorter than you would expect. Carefully read the Terms of Sale in any auction catalogue whether online or hardcopy. 

And that a Third Party Authentication is just an opinion, albeit a respected opinion. You must also always get a guarantee of authenticity from the seller because a Third Party Authenticator is not involved in the actual sale.

All the above also applies to buying directly from a dealer.