How do logged in users post their products on the main site

Want to create an etsy type site where people setup accounts and post products on the main site.

I have setup multisite, have activated marketpress, and I know how to post products on my own. How do users or people who have created sites under my domain post products that end up on my site.

I even set up an account as a user to test how a user/site creator under my domain starts posting products that will end up on the main site. There was nothing for the user to post products.

How do logged in users or users who created sites post their products on the main site?

Please explain how thoroughly, thanks