First off all authenticators are human -- I have personally seen them flub ( I took an autograph book to (JSA) and they counted the wrong amount of signatures, in the hockey HOF book, I acquired -- mistakes happen -- great authenticators are (JSA, GAI (Tim Priddy), PSA/DNA, Richard Simon, Kevin Keating and if you have old A's & Phillies memorabilia (Ted Taylor of STAT Authentic) ; but as I was once told by an old hobbyist -- even the best are wrong 15% of the time. So unless you witness the signature/s yourself you can never be 100% sure -- so to feel comfortable in your purchase remember the following steps.

1. Do Some Research - Search for known exemplars of signatures on Google or Yahoo! to help protect yourself from forgeries or secretarial autographs.  Make sure the seller has a close-up of the signature in the auction description.  If there isn't a close-up in the auction, ask for one.
   
   
2. Look for the Money Back Guarantee - While a Certificate of Authenticity (COA) can be reassuring, they are easily made and many times are not worth the paper they're printed on.  Make sure the seller guarantees the authenticity of his/her items and has a return policy.
   
   
3. Know the Seller - Check the seller's feedback and history on eBay.  What do others have to say about their merchandise?  How long have they been selling on eBay? 
   
   
4. Ask Questions - Feel free to ask the seller any questions that you may have about the autograph.  An honest seller will have nothing to hide and should respond to your inquiry.