“Flipping”. My niche is electronics. Knowing market value of computers, phones, etc. I would buy locally and then post online. I also have a background in IT so I buy a lot of broken devices to fix and resell and even flip broken devices without repairing them at times. This really can be done with any hobby. The downside is that I source a lot of my items locally so it’s not just limited to online. Some things also sell better or for a higher price locally than if I list them online. It’s definitely a hustle but the amount of money you can make just by buying and reselling items is ridiculous.