1.  Google the autograph and you'll likely find it available in the public where you can look at it to compare the two...if they are IDENTICAL, that may be a reprint...and if they are VASTLY different (i.e. all of the letters are formed differently) and you have already bid on the item, politely ask the seller for his COA. If it is a piece of paper with the sellers name on it saying in effect "This is authentic." ask for more...if none is forthcoming, ask to be removed from the auction, and if less than 12 hrs are left, crooks will become non-communicative untill you win the bid and then they ask for the money. Contact Ebay and ask for a mutual withdrawal of the auction, where you pay nothing, and they get their fees refunded.

2. If the seller is reluctant to work with you, and tries to "persuade" you through the sale without your complete satisfaction as to the autograph's authenticity, you can remind them that forgery is a felony, investigated and prosecuted by the US Secret Service, FBI, and sometimes US Customs officials, if it is something involved in international shipping.  

3. Check your prices!!! If the cheapest you see an Eric Clapton signature is $3000, it is highly unlikely that you'll get his AND Ginger Baker's signature on a pickguard for $67, seeing as how Cream was just inducted into the Rock And Roll Hall Of Fame. No matter how badly you want this autograph, you must let common sense rule here....the crooks rely on people's desire for that great bargain that will make them feel so good, at least until they realize they've been hoodwinked.