Haven’t done it in years but used to make a nice amount selling used shoes, socks, and tights on eBay. Would literally buy a £4 pair of ballet pumps in Primark (the awful ones that stink after two wears), then sell them.
Saturated market and eBay clamped down so moved to another platform but then lost interest.
I’m no longer in need of an additional income, but so some content development for a few companies. I fell into this as my Instagram page got popular due to an interest of mine, so I started writing on the topic for other businesses.