At its heart, blogging is about sharing your knowledge with the world. Choosing a topic that you are passionate about makes the process of starting a successful blog so much easier. Writing about more than one topic is totally fine too. As long as you are writing about things that you are genuinely interested in, your passion will shine through and keep your readers interested.
Lets say, for example, that you refurbish and resell used smartphones out of your home. You could use a blog to attract visitors to your website where you list your current phones for sale. Your blog might cover topics about DIY refurbishing. On one level, it seems counterintuitive because you want people to buy your phones, but it also helps you build a brand and gain recognition. Social media guru Jay Baer explains the concept on Copyblogger: