Global products are not showing? Check this:

I took me one whole evening and the reinstall of my whole multisite network from scratch - but I finally found why the products on some sub-stores weren't appearing on my home global marketplace:

If a sub-blog is hidden from search engines, the marketplace stops displaying its products!

It's a nice touch, but as it is undocumented, it made my whole test enviroment seem broken.

So, if everyone stambles on this same problem, take a look at the general settings of your sub-blog- it may be hidden from search engines and its products also from marketplace!