Team leader Paul Eremenko said in an interview at Purdue University that there will be a special store for Ara components, complete with recommendations and reviews.
This will emulate the Google Play Store and its software sales, but for the modular hardware that will make up the Ara.
He also reiterated that the Ara MDK is free and open and available to everybody. So presumably anyone could create a module, just like theoretically anyone can make an Android app. If you do that, you then put it up for sale in the Play Store - while for Ara hardware modules you'll be able to do the same, using the upcoming Ara module marketplace. This will allow module makers to sell them directly to consumers, which is apparently what Google wants.
It will be interesting to see how many different modules will be dreamt up by developers, and how many the Ara module marketplace will stock when it launches. In the mobile software world, for a long time success was measured by how many apps were available for a platform. So if Project Ara wants to emulate that, then it should have a lot of modules listed in its store.
Project Ara was started at Motorola, but Google kept it when it sold the phone maker to Lenovo. The base price for the modular smartphone will be around $50.