Leapforce is always looking for people for this work at home role. Their Search Engine Evaluators “conduct research, evaluation, and feedback on search engine results by measuring the relevance and usefulness of web pages in correlation to predefined queries, by providing a comparative analysis of sets of search engine results and various other techniques.”
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.