With a world’s worth of secondhand and used clothing available, resale platforms like Depop are connecting buyers and sellers of even the most niche and specific styles across large distances.

This as-told-to essay is based on a transcribed conversation with Danielle Mass, a 25-year-old founder from London, about starting a Depop shop. It has been edited for length and clarity.

Secondhand fashion has become one of the best ways to make a quick buck. Apps like Depop and Vinted provide a platform for side hustles and wardrobe spring cleans alike. Where consumer demand ...