Unfortunately, bogus signatures abound on eBay as well as other venues. The best way to ensure that you have an authentic signature is, of course, to have it signed in person, but this is just not possible for most of us. So, if you work for a living :) or live far from the large cities where signing events happen, please consider the following:

• If an item is being sold significantly cheaper than other items of the same description (especially with a “buy it now” sale), it is usually a fake. If it’s too good to be true, it probably is too good to be true.
• If a seller has many copies of the same autographed item, be careful!  Most book signings have a limit as to the number of items that will be signed. All those I’ve attended had a limit of 2 to 10 books, depending upon how in-demand the signer was.
• Be careful about COA (Certificates of Authenticity) which are often photocopies; not a valid way to authenticate an item.
• Look at the seller’s reputation; if there is a high number of positives and/or the seller has been on eBay for a long time, the seller is doing something right.
• Check the seller’s previous sales (not just the feedback); click on some of the actual auction numbers to see what the recent sales history is. If a person is selling (or has sold) many copies of the same item, be careful.
• Ask questions! You have a right to know the “history” of, or the story behind, this item. Ask how the seller got the signature. If the seller is unresponsive, you may want to deal with someone else. 
  
• If you believe you have purchased a fraudulent item, report it to eBay's Fraud Protection Program and PayPal.
• eBay is NOT an online store; it is basically an online flea market – use common sense. 
  
• Remember the caveat:  buyer beware.