Sitepoint is the online marketplace by excellence. Some websites and blogs, however, are trying to replicate that model on a smaller scale.
Depending on your niche, a market place that allows your visitors to buy, sell and trade products could work well. Over the time you could start charging a small fee for new product listings.
The problem with this method is that there are no standard software on the web, so you would need to hire a coder to get a marketplace integrated into your website.
You can see an example of a marketplaces being used on EasyWordpress and on Mashable.