Amazon dominates the ecommerce market in the US. This year, retail ecommerce sales are expected to reach $525.69 billion, with Amazon claiming nearly half of that total—up from 38.3% in 2016. The remaining top 10 ecommerce companies, ranked by digital sales, will account for 21.7% of the market in 2018.

The success of Amazon Marketplace, where third-party sellers can sell new or used products alongside Amazon offerings, is a key factor in the ecommerce giant’s growth. We forecast Amazon’s marketplace will account for 68.0% of Amazon sales and 33.4% of total US retail ecommerce.

Sales on competitor marketplaces trail Amazon, as they attract a lower number of sellers (and in some cases the same sellers). According to a May 2018 survey from Amazon intelligence company Feedvisor, for example, 52% of US Amazon sellers also used eBay’s marketplace. The overlap in the number of sellers on Walmart and Jet.com was much lower, at 17% and 8%, respectively.

We estimate that No. 2-ranked eBay will lose market share for the second straight year, falling from 7.1% to 6.6%. The scale of Amazon’s marketplace means it’ll capture at least 30% of ecommerce sales across each major product category (except auto and parts).