What is JetBrains Marketplace?
JetBrains Marketplace is an evolution of JetBrains plugins repository (https://plugins.jetbrains.com). It's turned into a marketplace for third-party plugin vendors (plugin developers) where they can sell their plugins taking advantage of licensing and billing solution provided and maintained by JetBrains, in addition to currently available plugins repository for free plugins.
In a nutshell, JetBrains is providing:
Licensing solution by JetBrains, integrated with all JetBrains infrastructure (similar to how it works for JetBrains itself - licenses generation, support for trial and evaluation licenses, licenses via JetBrains Account (JBA), license server, centralized licenses administration/management, licenses protection, special API in the product to check that user has a license, etc.)
Billing and sales infrastructure, and a distribution channel provided by JetBrains (includes payment processing, resellers and distributors network, sales support by JetBrains for end-user invoicing, refunds, etc.)
Marketing and sales support (various activities to promote plugins via JetBrains or standalone channels.)
There is a moderate commission for plugins sold via the Marketplace.