Ebay is a bellwether. It's been around since nearly the start of the online boom. But, like any other platform, success can seem fleeting if you don't know what you're doing. Selling items on eBay, professionally that is, can be an art form. Getting people interested in your auctions isn't always easy, especially when there's hefty competition and low demand for what you're selling.
Research individual companies in your desired niche: If possible, it’s always better to become an affiliate directly with a company (if they have an internal affiliate program), as no one else will be dipping into your commission rate. This is the preferred route for most of the prominent affiliate marketers, including Pat Flynn. Unfortunately, it’s also the most work, as you’ll have to do the research yourself to see who offers programs (they’re usually listed in the website footer).
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.
If you live near a university, there are likely all sorts of research studies looking for participants. While I was an undergrad at Virginia Tech, I got paid $500 to participate in a 6-week dietary study. The study provided all my meals and paid me, but I had to eat a 5,000 calorie diet of 50% fat for 6 weeks, plus I had multiple muscle biopsies, urine/blood testing, etc.
Outsourcing is the new and dominant wave of the business world. As businesses shed employees, they increasingly turn to independent contractors to supply what in-house staff used to do. One of the ways they do this is through virtual assistants. These are people who provide certain tasks online. Businesses can use virtual assistants as much or as little as they need to. If you can make yourself available to multiple businesses, you might have plenty of work without ever leaving your house.
This has become a popular business model for online entrepreneurs over the past several years, and will probably just continue to grow in popularity. The best thing about selling online courses is that once you do the up-front work in creating the course and setting up your marketing strategy, you can get paid over and over again for work you do once.