An online marketplace is a type of ecommerce site that usually is built up of multiple retailer and third party products that use the site as a way of retailing a product or selling a service. However the transactions are managed by the site operator. A perfect example of this is Amazon which other people can use to market their items. Amazon is a big example of a marketplace as they also sell their own products and services.
You also have the option to be able to match marketplace products if you have asked us to track the likes of Google and/or Amazon.