How to setup a Facebook shopping cart
http://tomuse.com/sell-on-facebook-ecommerce-shopping-cart-storefront-app/
http://inchoo.net/ecommerce/how-to-sell-on-facebook-shoplets-and-fan-pages/
Why not to
http://seewhy.com/blog/2011/03/28/forget-selling-on-facebook-for-now/