Buy underpriced used books. Outfit a phone with an ISBN reading app, scan the ISBN numbers of books at used bookstores and thrift shops, and compare the asking prices with what the books are selling for on a site like Amazon. Whenever you get a good hit (which won’t be often but, since the process is fast, won’t take long, either), buy the book and resell it online. Be discreet about this, as the store managers probably won’t like what you’re doing.
Great article, I have tried a few over the years. I had a side business cleaning offices for years. It did great, but I hated never being able to take more than 3-4 days vacation, so I eventually gave up. I tried MTurk, but if you multiplied my earnings on an hourly basis, it was a little over $6/hr. My blog is much worse, probably less than $0.25/hr.