Ebay, Amazon and even Alibaba and its variants have revolutionised the way we purchase items and the speed at which we are able to have them delivered. It truly is about the convenience and the hassle-free way of living.
With blockchain tech, we can expect a significant disruption with these ecommerce giants as it will innovate how quickly, efficiently and with a great level of security that we can conduct online transactions and purchases.
E-commerce fees and percentage margins are way to big. They can really charge this because they are the source of validity for the transactions. However, blockchain will flip that upside as each node is now a verifier of the transactions taking place. Hence, this eliminates the entire need for a centralised 'giant' to validate each parties' transactions with each other and with the business.
I am glad you brought up ebay, amazon and alibaba because they are the one's that will be most affected by blockchain in the ecommerce industry.
Check this infographic, it is eye opening to see (hopefully the stats are correct):
https://assets.econsultancy.com/images/0001/1382/online-shopping-around-the-world1-e1301325718539.jpg