Percent of worldwide units sold by marketplace sellers.
As the marketplace share rises, the opportunities for sellers rises too - Amazon overall growth is directly available to marketplace sellers.
Amazon first introduced third-party sellers back in 2000. As Brad Stone wrote in The Everything Store:
Lower prices led to more customer visits. More customers increased the volume of sales and attracted more commission-paying third-party sellers to the site. That allowed Amazon to get more out of fixed costs like fulfillment centers and the servers needed to run the website. This greater efficiency then enabled it to lower prices further. Feed any piece of this flywheel, they reasoned, and it should accelerate the loop.
Last reported quarter 2017 Q4 it was 51.0%, up by 4% year-over-year from 49.0%.
Source: Amazon Quarterly Results.