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.
This is the best way to do ,as he mentioned ebay and amazon doesnt accept directly bitcoin as payment . Since it isnt available as of now then the last best way is to convert directly you bitcoin to fiat by your local wallet provider you could able to exchange it.
As far as I know there are ways to buy in amazon using Purse.io but I have not tried it yet because I have a big problem with the shipping fee in our country, it is too expensive and I was thinking that I would rather cash out my bitcoins and my in our local store physically.