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?
Which country do you live? It quite easy to sell your coin, there are plenty of exchangers that could be convert your btc to fiat money.
Usually, for beginners recommended to join bitcoin community (there is in every country) that can help you about this, just search it in Facebook, you can get there.