We just released Nuxeo Platform 5.9.2, a Fast Track version. One of the new features is a private channel on our Marketplace. We made a short 4-minute video to show you how it works. And if you prefer reading, you can take a look at the documentation.
The private Marketplace channel is available to any Nuxeo Online Services user. Nuxeo Online Services trial users can also use it. It works on Nuxeo Platform versions 5.8 and greater.
About the Marketplace package: it contains simple XML configuration, Java code, configuration templates, libraries, etc. All you have to do is respect the Nuxeo Marketplace package format.