How do multisite marketpress users post products in etsy type site

creating 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 users or users who created sites on my site post their own products and it ends up on the main site?

Please explain thoroughly, thanks