I am currently Jumping through the hoops in setting up chained PayPal (it is a struggle - but I am in no real rush to get things running).
Anyways... while messing about with initial testing I thought about a Feature that could be included within a future version of MarketPress.
If one of your users is in the progress of setting up a store, they would select 'SandBox mode' - this is an ideal time for them to set up their store and test things out - but their products still show up on the main Network Site.
They can easily change their privacy settings in WordPress to stop their items appearing sitewide but I think it could be a nice addition to FORCE the privacy setting when they have a SandBox enabled.
This will stop products appearing on the 'flagship' site when the users store is not quite ready!
I figured I would share this suggestion with you all!