When Levine, Locke, Searls and Weinberger told us in The Cluetrain Manifesto (1999) "Markets are conversations" we should have readily seen the corollary would not be far behind: Conversations become markets.
The first web marketplaces democratized the ability to sell physical goods, such as E-Bay (founded 1995) and Etsy (founded 2005).
Amazon, started in 1995, launched third-party selling as Zshops in 1999. That change, along with services to independent publishers, really made Amazon a marketplace rather than just a retailer.
Digital goods marketplaces got going with photography. The one that comes to mind (there were likely others) is iStockPhoto, started in 2000. Typography, icons, and other graphic elements are all now available in marketplaces such as Creative Market (founded 2011.)