I don't understand why anyone feels there is any value or notariety within this 'idea'. This is nothing more than a meager marketing tool, the only difference is they take a massive chunk of your profits, sound familiar? Ever heard of ebay? The finders fee is an age old technique used by the smart to pray upon the weak, lazy and stupid. Everyone understands the value of good marketing. However, ebay, craigslist, paygr, google, yahoo, and any other listing website does -not- do -any- of the marketing for you. It simply makes the information you made slightly more accessable to others, if at all.
If you want the biggest bang for your buck, I suggest looking into CMS (content management systems) like drupal, joomla and wordpress. These CMSs all have fantastic user bases, extensive plug-ins and they all do what everyone recognizes on the web as websites. Make a website and use one or two keyword based search engine enhancers such as adwords. The only other things that are going to give you a bigger 'bang for your buck' involve solid and awesome products or ideas aligned with excellent customer service.