WP Marketplace is all, what you'll need to build your own marketplace. WP Marketplace is packed with all necessary features you must need to build a e-shop for your single product or a full featured complete market place like codecanyon.net or themeforest.net with very easy management. WordPress Marketplace plugin is compatible with any WordPress theme. You don't need to change anywhere after your install. It's just ready from all side for your use.
Plugin info page: http://wordpress.org/extend/plugins/wpmarketplace/