My 10-year-old son brought home a book from our park’s free library box. It was a biology textbook – teachers edition. He said it looked interesting and hey, it was free (having no idea you could sell it). I scanned it in my Amazon seller app and realized it was worth around $150. He was so excited. We listed it for sale for $130 and it sold! Going to tell him, he just made $130!
Our first overlooked opportunity is selling your knowledge. In this instance, we are talking about creating a course or eBook that you can make available for purchasing online. Because of the automation available today, you may be able to put in the work once when creating your packaged information and then enjoy those checks coming in for months or years to come.
If you have experience and knowledge in a specific area, then consider sharing it with others. For example, if you’re an accountant or lawyer, then you can provide advice to small businesses for a pretty penny. You could also consult businesses on how to use a new software program or how to become more environmentally friendly. (If you're interested, my company offers a consulting guide to get started.)
Find a profitable niche. Starting with your interests, write down as many niche ideas as you can. Think about topics people might search online. Ideas include passions (like surfing or body building), fears (like spiders or speaking in front of crowds) and problems (like getting out of debt). Do keyword research to see it others are interested in the topic. Find out if a domain name is available that matches the keyword 100 percent. 
Do you love getting refunds? How cool would it be to get money back on stuff you’ve already bought? Paribus is a service that lets you find out if stores you’ve shopped at online owe you a refund. It’s free to sign up. Paribus connects to your email account and checks your receipts. If they find out a retailer has dropped their price they file a price adjustment claim for you. Try out Paribus.
Here’s a good example of how lead sales can work in real life: My second website, Life Insurance by Jeff, brings in a ton of traffic from people who are searching the web to find answers to life insurance questions. While I used to have the website set up so I could sell these people life insurance myself, it was a lot of work to process all the different requests and clients. As a result, I started selling the leads I gathered instead.
I also participate in online surveys through the following websites, that work in the US and internationally (I use them all and I'm based in Brazil, for example) - Opinion Bureau, Myiyo, and Green Panthera. All of them have a variety of surveys and pay well. All you need to make money is to have 20 min or so to answer a few questions every day/week and have an active Paypal account so you can cash out.
Set up an account on Rover.com. Enjoy furry company once in a while? These days, many professional pet-sitters set up an account on Rover.com. With Rover.com, you can reach people in your area who are actively looking for someone to watch their dogs, cats, hamsters, or turtles, either in their home or your own. You can set your own rules and schedule and come up with your own pricing through the Rover.com pet-sitting platform.
If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!