Context - This is a proposal from Google based on our experience consuming Schema.org markup and working with similar data from online merchants. If it were accepted, it would make it easier for us and others to understand online marketplaces.

Proposal - An online marketplace is a collection of online stores, which are independent 3rd party sellers offering their products on the marketplace. The marketplace itself can also have its own online inventory, which it is selling alongside the inventory of the 3rd party sellers. Schema.org currently only models individual online sellers (through the OnlineStore type), but not online marketplaces. We therefore propose to add a new type OnlineMarketplace (as subtype of OnlineStore), with a new property hasStore (and inverse property isStoreOn)