Amazon.com Inc. continues to strengthen its dominance in U.S. ecommerce thanks in large part to its third-party marketplace sellers. Sales from Amazon’s marketplace account for roughly 60% of its total gross merchandise value, Digital Commerce 360 estimates. In Q1 2021, marketplace merchants accounted for 55% of units sold on Amazon.com, according to the retail giant. That’s up from 52% in the same quarter of 2020 and 44% five years earlier in 2015. Amazon is No. 3 in Digital Commerce 360’s Top 100 Online Marketplace ranking.