It’s one of the oldest and most proven ways to make money – buy low, sell high. The buy low part comes from searching garage sales, estate sales, and even thrift stores to find items that are in good condition (“gently used”) but selling well below what they would if they were brand-new. In this way, you might be able to acquire an item for $5, and later sell it for $50.
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.