The Marketplace is an Audience Marketplace feature for data buyers that lists data feeds you can subscribe to. It lists flat rate, CPM, or private data feeds. These feeds are provided by third-party vendors that use Audience Manager to sell data. In the Marketplace, reporting tools let you track feed usage and the overlap between your traits and those in a subscribed data feed. Finally, with Audience Marketplace, Adobe takes care of invoices and fee payments (though you do have to self-report usage when subscribed to a CPM feed). These features let you find effective data sources without wasting time looking for a data provider.
In the Marketplace list, sometimes the provider's name and trait data are market as private. This indicates a private data feed. A private data feed lets sellers limit buyer access to their data. Sellers can make feeds private when they're offering special deals, discounts, or when privacy and access control are important to them. As a buyer, you have to send a subscription request to the seller if you want access to a private feed. See Subscribe to a Private Data Feed for details.