multisite marketpress question

Not totally sure how to phrase...
I've set up my network and created only my mainpage on my main site (main site to be like etsy housing other users sites). This main page will have a grid of boxes with items/products that are pulled from the other users stores.
1)How do I link to other users products from their stores globally on my main page?

2)How do I set up these boxes to pull randomly( either daily, or by each separate hit on the main page)
from my other users stores?

Thanks,
kate