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.

The news that Depop – Generation Z’s favourite app for selling and buying used clothing – had been sold to Etsy for $1.6bn (£1.1bn) is a warning shot for fashion retailers. For years, traditional ...