It’s something akin to picking stocks. You want to buy undervalued domains, and sell them later on at a higher price. For example, you can pick a domain that is out of favor, but could be related to some future event. So if you decide that the stock market is likely to crash in the future, you can buy a domain that includes the words stock market crash during a rising market, and then sell it in a falling market.
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. 
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