I don't think that you would be able to buy anything directly using bitcoin on ebay or amazon as they have not yet started to accept bitcoins as a mode of payment so then only option left for you is to sell them and exchange for fiat and buy the using COD method.
Ye but that's the thing. Where to sell my bitcoins.. Any good site recommendation?
It will depend on the country that you are living, I am from India and for me the best place to sell my bitcoins is on localbitcoins as there are trusted traders and will transfer fiat to your bank account within 10 minutes and it goes risk free.