Wordpress Network - E-commerce Global Items

Hi! I am trying to run Marketpress on a wordpress network. We want ALL of our stores to be able to sell most items and also be able to sell their own custom items.

We don't want to have to update every store when an item needs to be updated across the network. We could do this with lots of import/export in MySQL, but we're hoping to find a plugin that does this easily for us, as we have 150 stores already.