This hasn't been asked yet so I'm going to bring it up. Integrating MarketPress with the BP Achievments plugin seems like a natural feature that should be included in a future version of MP (though I'm not trying start feature creep!)
This would be a great way to reward and recognize outstanding sellers. Paul Gibbs has a plenty of into on adapting plugins to Achievements for this exact sort of thing http://achievementsapp.com/developer/add-support-for-a-plugin-non-custom-post-type/
A good implementation of this would be able to unlock achievements for items sold by a seller, and also (if a buyer has an active account within the BP site) the number of purchases made.
MarketPress is already leading the way in WordPress e-commerce with its support for Multisite, as it gets further integrated into BuddyPress having support for Achievements would help maintain that edge.