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.
It’s sometimes hard to comprehend just how much people love t-shirts. And with the right niche, marketing and tools, you can create an online t-shirt business that makes you extra money online while you sleep. (Even Bloomberg and Forbes feature stories from entrepreneurs who've done just that.) Services like TeeSpring make it easier than ever to create a t-shirt drop-shipping business where they handle the sales, printing, and shipping, and you’re only responsible for design and marketing. Or you can even buy into well-established franchise businesses like Rhea Lana that sell fashionable, affordable kids clothing and get the backing of a proven organization to fuel your online sales. For more tips, check out this simple guide to launching and marketing an online clothing store by my friends over at Selz.
Mechanical Turk: Amazon's Mechanical Turk is a resource for doing human-intelligence tasks, or as the site commonly refers to them, HITs. You get paid a very small fee for any given HIT and you'll need a good deal of volume to make a substantial amount of money. But it is a resource you can use in your spare time to generate a small income online. 
Walk around your neighborhood or town and I’m sure you’ll see tons of great local businesses with terrible design. However, with increasingly easy-to-use tools like Adobe Illustrator, Venngage, Stencil, and Visme, just about anyone with a creative mindset and a good amount of motivation can start making money online by being a graphic designer for local companies.
Starting a podcast, like making a YouTube channel or blog, comes down to telling interesting stories and building an engaged audience. I’m probably sounding like a broken record by now, but you need a niche that you’re interested in and there’s already a demand for. Come up with a list of topics you’d like to talk about and then search iTunes charts, Google Trends and other podcast research sites like cast.market to see what’s currently out there and popular.
Find a profitable niche: We’ve talked about this a lot. But, where are you most comfortable. What niche do your skills, values, and interests intersect? Do you have 10 years of experience as a technical writer? Do you have long-standing PR relationships that’ll be invaluable in helping startups launch a successful crowdfunding campaign? Determine what makes your value unique, and lean heavily on showcasing that strength to your potential clients.
×