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!
Wash cars or bikes if you have a big enough driveway. Gather some sponges or old rags, towels, car washing liquid, a big bucket, and a hose and set up a washing station in your driveway for a few hours on the weekend. Scrub each vehicle, focusing on any dirty spots, rinse it thoroughly, and buff it dry with a microfiber towel. Charge $10 to $20 per vehicle.
Regardless if you need to earn some fast cash or we're just talking about making money in the grand scheme of things, there's an important psychology that needs to be mentioned before getting into the strategies. If you study Freud's model of the mind, you'll discover the Psychic Apparatus. It's the three-part construct in your mind that controls all of your behavior.