marketpress checkout

I had a main site with 3 sub-site.

what I want to do is something similar to etsy like what marketpress promised to sell like etsy.

when i try to buy or add to cart, it always go to the subsite.
in ETSY, we able to checkout the product one by one from the main site as we can view our cart from different site.

Here, I can't see the product I added in sub-site in my cart.
Am I missing something on the setting ?
it kind of troublesome if I wanna buy a few thing and keep shopping then only go to checkout, but if in my global cart, I do not have the information, i couldn't able to remember which are the site i buy from for the item been added to cart previously.

Please advise.