This is copied from callawaygolf.com.
"Consumers from the United States, Austrailia, Europe, and Canada have recently purchased sets of so called "Brand New Callaway Golf Clubs" at very low prices on ebay.com and other internet auction sites.  These clubs were in fact low quality-fakes.  Callaway Golf's authorized retailers are not permitted to sell new Callaway golf clubs on ebay.com." 

These counterfeit clubs are usually bundled as a complete set, still in the plastic, with a bag and putter.  Most of the counterfeit clubs I've seen for sale on ebay are in a black Callaway bag with a green chevron logo.  The green chevron logo is copied on the headcovers of the woods.  The headcovers themselves are furry, tube-like headcovers.  The putter is also called a Callaway model, and has a face insert that says Callaway on it.  As far as I know, Callaway doesn't actually make a bag with a green chevron logo like this,  they certainly do not put their name on headcovers as cheap as these, nor do they make a putter with a face insert in it that has this logo.  These clubs are usually offered by sellers with little or no feedback, and the seller certainly doesn't take closeup pictures of the clubs.  You would know they were fake because of their poor paint jobs.  Before bidding, ask the seller for the serial number on the on the heel of the driver, 3 wood, or 8 iron.  If the seller doesn't reply, refuses to give you the number, or states that the number isn't there, you can assume there is a problem.  Ebay and Callaway are currently cracking down on these listings, but they still show up from time to time.

What this means is that if you want NEW CALLAWAY GOLF CLUBS, you should visit a retailer.  If you'd like a nice set of USED CALLAWAY GOLF CLUBS, I recommend that you buy from a powerseller who has a lot of positive feedback.  I register the clubs I sell with Callawaygolf.com, so I can guarantee that they're authentic.

Use common sense when buying on ebay.  If the price is too good to be true, it probably is.  Buy high-end items from powersellers who have plenty of positive feedback.  Check their feedback and see how they have handled problems in the past.  Check their return policy and check their feedback for people who've successfully returned items.  Avoid sellers with little, no, or negative feedback.  Avoid buying from sellers who don't answer questions, as this will show you the kind of communcation to expect after the sale.  Also, avoid sellers who only deal with money orders.  This is just like sending cash in an envelope.  There is very little protection for buyers who pay this way.

I hope this guide helps you in your endeavors on ebay, and feel free to ask me questions or make comments about this guide.