Multisite products and posts to display on main site

First off the support here on WPMU-DEV is very good. I usually get a helpful response very quickly, thanks.

Anyway, I am pretty new at web-design so please bear with me.

I'm getting to the point to where I'm wanting to open up registrations for multi-site. My question is: once site owners start posting products/posts how do I get those to display on the main site? Similar to Etsy, Amazon and others?
I've read through the manuals and couldn't find anything about that.

Thanks in advance for your help.