Finances. You need to be good at quick math and be able to make quick decisions. You have to be careful how much you pay for a book, and then keep careful track of the expenses involved in selling it. For instance, when you sell on a website, they’ll take a commission from the sale. In addition, some sites, like eBay, will charge you a listing fee for each book. If you request that your money be deposited via PayPal, then you’ll be charged a transaction fee per book. If you’re not careful, the fees will quickly negate your profits.
Research other listings in your city on AirBnB and see what the going rate is for a place like yours. You could also just rent out a private room as well or even a bed in a shared room. In fact, that's how AirBnB got its start. However, you might find it hard in the beginning without reviews, but as long as you take really good care of your guests and provide a lot of value, the reviews will eventually come rolling in.
Like talking to people and helping walk them through problems? You can bring in extra income as a contract customer support superstar for companies all over the world. Due to the 24/7 nature of online businesses, companies are looking for people in different time zones to help deal with issues that their users are having. Better yet, if you have experience in service or retail you’ll be perfectly suited. Check out indeed or the other remote job boards I listed earlier to find opportunities.
Can you name 30 products related to your topic? If you know your topic well, you should be able to list 30 or so related or complementary products or services. Can you? If you can, great. There’s a good chance you’ll find affiliate partners that sell products that you can promote in your content. If you can’t name 30 related products off the top of your head, your topic may not have much profit potential.
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.