What’s the best method to add a Q&A section to an e-commerce site?

Use some kind of a registration / login process, OpenID, Twitter/FB connect for users to have before they can post answers. Initially you could moderate all Answers before they becomes visible on your board (a tiring effort, but one that is worthwhile).

Eventually you would need moderators who do this work for you, who will patrol the boards so to speak and discourage and delete such spam filled posts.