Thanks for writing this Mr. Samurai. I just got over the student loan hump but I feel pretty good about it at 27 having a graduate degree and being 100% debt free. Now that I’m on the other side it is good for my brain to absorb some of your knowledge regarding passive income investments. I love gleaning wisdom from older folks who have been there and done that. Mentors rock!
Speaking of selling stuff online, that's another mostly passive way to generate income. You could generate an income stream for a while by clearing out your basement or attic and selling items on eBay or elsewhere. This can be especially effective with collections. If you have lots of games or jigsaw puzzles that are taking up space and not being used, they can be great sources of income. You might reap a lot by selling new and used clothing you don't need.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.