Beware of autograph sellers offering a "Certificate of Authenticity". They are absolutely worthless. If you will fake and autograph you will fake a COA.
Most reputable autograph dealers will not offer COA's but instead offer a lifetime unconditional guarantee. If you are unhappy, simply return the item is the condition it was sold to you.
You will see sellers offering guarantees that run out in 7 days, 10 days, 30 days, etc. That should be unacceptable to you as a buyer.
Some sellers will state that a questionable signature cannot be returned without it being authenticated by an expert. They know that this is VERY expensive to do. To get a signature authenticated will often run in to hundreds of dollars. These sellers know that no one will go to that trouble for an autograph that was bought for under that amount.
The Universal Autograph Collector's Club (UACC), one of the most - if not THE most respected autograph organizations in the world, have long renounced the use of COA's. Here are a few FAQ's straight from their web site:
Q: Are Certificates of Authenticity endorsed by the UACC?
A: The UACC has been very vocal about the fact that Certificates of Authenticity (COA's for short) are totally worthless. The UACC does not issue COA's and always encourgae collectors to get a signed receipt for merchandise, instead of a COA. Frankly, they aren't worth the paper they are printed on. A COA is only as good as the dealer that has issued it. If you buy from a UACC Registered Dealer and get a receipt, you are following the correct procedure.
Q: I was looking at some items on Ebay, and I saw that an item came with a UACC Certificate of Authenticity. Is that possible?
A: No it is not. The UACC does not issue "Certificates of Authenticity" The person selling that item should immediately be reported to Ebay.
Just use common sense when you purchase autographs. Ask the seller questions. Ask about their return policy. Learn these things up front so you will not be disappointed when your signed item arrives to your doors.