WordPress Marketplace is a free full-featured Shopping Cart / eCommerce Plugin with everything you need on an easy UI, to build eShop or marketplace.
Not sure why this has such high reviews unless it only recently broke. The checkout process is a mess of messy code. PHP errors on a fairly standard install, js errors and a half-finished user creation script. Will be great when it's a little more polished. A word of warning, you'll might need to enable sessions if you're on a cheap host.
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