MarketPress was developed to be a good solid, simple, extensible foundation. It is the first e-commerce plugin designed from the ground up for seamless integration with WordPress Multisite and BuddyPress. Much work went into detailed but simple Shipping and Gateway plugin APIs, and very robust themeing support with it’s own set of completely documented template tags. Also it makes full use of the custom post types, taxonomies, etc. of WP 3.0 so it creates NO messy custom tables for each blog that crowd your DB like all the other shop plugins out there!