I am a serious collector of guitarist autographs and have acquired a few on Ebay.  I first developed an interest in the hobby after reading a book on handwriting analysis in the 80's.  However, in order to buy autographs on Ebay, you have to really educate yourself and become jaded, as such a large number of them are indeed fakes.  So how can you tell?  The first big tip-off is when the seller uses a private auction.  I will NEVER bid on one of these.  Secondly, a big sign is when the seller has many other hard-to-get autographs at the same time.  Thing about it: you know how hard it is to get and maintain a big inventory of autographs?  It's next to impossible, and it's the reason why there are so many fakes: a person would have to sell them for more money that is possible to cover all the time it actually takes in proccuring those autographs in person.  I know!  I've spent many hours waiting outside venues and soundchecks to get them!  If I was paid $20/hr to do it, I could never recoup my time in selling them.  It's the economic reality!  As for looking at the feedback of the seller, I find that mostly worthless.  The reason for this is because it just shows that the buyer received the item they paid for.  When they learn it's fake way down the road, there's no possibility of redoing the Ebay feedback!

It's very valuable to study a person's autograph before trying to buy one.  One of the hardest thing to fake about an autograph is the "flow"... how the line flows into each letter without interruption (look for any spots where the pen stopped!).  That "flow" is tough to fake.  As a resource, take a look at www.rrauction.com and view past auctions to see examples of authentic autographs from a reputable source. 

Be especially aware of easy to forge autographs, such as Eric Clapton.  By far, most of them offered on Ebay are fake.  And COA's mean nothing to me.  There are many less fake autographs on something of value, such as a guitar or a tourbook.  Because if the forger screws up on the first pass, the item is worthless.  Realize too that a guitar isn't really "signed" if it's only the pickguard!  At least, that's the way I look at it.  I get pickguards signed myself, but don't go and stick them on a cheap japanese guitar and say the guitar is signed.

Lastly, as with everything in life: if the price is too good, it probably is!