questions regarding multi-site implementation

I am referring to this quote from the plugin description:

And, did you know that MarketPress is also 100% WordPress Multisite and BuddyPress compatible – so you can use it to start your own Etsy or Ebay style network of stores… where you take a % on each sale!!!

I am very interested in this feature but I need to understand who this works:

Lets say I have a main shop: mainshop.com and two other sub-shops: shopa.mainshop.com and shopb.mainshop.com

I set up my payment gateways on the mainshop and the commission and other settings.

– How does the commission story work?

– Does shopa and shopb also need to set up their respective payment gateways or are their shops using mine?

– How do I get my commission?

– Does this commission story work with every payment gateway that marketpress supports?

Additionally you can answer the same question for your Events plugin and the Membership plugin too, since I understand all of them offer the same features?