There is no demand for it yet, becaue the majority of people doesnt use bitcoin.
We need to have at least 20-25% of users demanding bitcoin shopping before big retailers integrate their payments in it.
And for that we need bitcoin adoption. My tutorial in my signature is exactly designed for that!
There's a demand I believe but for the numbers Im not sure.
And no I don't see adoption is the main issue here. Even when there's a big adoption it doesn't mean they will be all using Amazon or Ebay.
With the current numbers of people using Ebay and Amazon, that is enough to make an excellent request to add bitcoin as their one of the payment option.
But IIRC I I think there was a third party so that people can use their bitcoin to the stated merchants above. Im not that sure.