A lot of carts have built in affiliate/referal systems that can be a great way to market your site. We choose to use an affiliate system for our marketing. The affiliate system still creates inbound linking for us from tons of sites that are already receiving traffic. So hopefully enough of it is quality traffic that the inbound links will help with other types of algorythms to determine our page listing order and what not. We set up two offers one per lead and one per sale. Each offer has 12 banners that users can choose from and copy/paste the code that already dynamicly contains their referal id tag and offer id tag in the URL. Its a really great system and it wasn't that hard to set it up. The best part is everyone wins you get paid, affililates get paid, surfers find offers worth buying.