Manishstores
First off welcome to Ebay and also be ready to face a lot of headache while selling on ebay.
Understand one thing, ebay is not here to support sellers they are here for the buyers.
Now that the buyer has raised a claim, call the customer care support and ask them what has the buyer written in his claim and then give your reply.. they will update your response on the claim. Next if you are lucky then they will call the buyer and have him ship the product back to you or if the claims department is lazy enough then they will refund the buyer as well as pay you the same money under "Seller Protection"
Since the Ebay guarantee has started a lot of buyers are using this to buy free stuff. Now dont go and fight with ebay or the buyer cause then he will give you a negative feedback which you dont want.