Diligence. In this industry, the line between scams and honest businesses is blurry, so you need to do your homework before accepting a position. Call the Better Business Bureau in your area to find out if there have been any complaints filed against the company, and do an online search on their name. Alpine Access is a well-respected company in the field, and Call Center Careers is a great place to look for legitimate call centers looking to hire.
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.