One of our Awesome users @aristath have cooked up a handy Statistics plugin for MarketPress. Check it out: http://wordpress.org/extend/plugins/marketpress-statistics/
Works like a charm. See the screenshot attached of my store.
The release post is here, do not forget to send him some points if you find this useful. You will! ( oh, also give it a 5 star rating on WordPress.org will you?