In the US eCommerce market the top 3 online stores amazon.com, walmart.com , and apple.com account for 73,080 million US$ revenue in 2017. Regarding the market volume they achieve a market share of 22% which means that the market is already concentrated

The eCommerce market encompasses the sale of physical goods via a digital channel to a private end user (B2C). Incorporated in this definition are purchases via desktop computer (including notebooks and laptops) as well as purchases via mobile devices such as smartphones and tablets.
The following are not included in the eCommerce market: digitally distributed services (e.g. travel tickets), online stores dedicated to digital media downloads or streams as well as online stores dedicated to B2B markets nor sales between private persons (C2C). Market volume, market growth and all monetary figures at store level refer to the annual eCommerce net sales after the deduction of returns.