Considering my site will talk about comparing products, and first-hand accounts of rip-offs, scams and fads, how would you protect the site from libel lawsuits?

Ensure to have a link in the footer of all pages that all posts in the forum are the opinions of the posters and not necessarily those of the site / forum owner. In addition, a Contact Us form saying: "The posts here reflect the opinions and beliefs of our forum members. However, we understand that sometimes false statements about a company, product, or service are accidently published due to misinformation, being uninformed, etc. if an incorrect statement about your company has been posted, please provide us with the thread URL containing the false statement, as well as a link [on your company's website] to where we can find information proving the statement is false, and it will be removed."

and remind people to post their OPINIONS - and mention that in the footer, that all posts are the opinion of the poster and not of the site owner