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.

Mika Kol, a.k.a., @TrustFundGoth, has 17,000 followers on the popular clothing resale app Depop. The 21-year-old business major is an enigma on the Instagram-look-alike platform, breaking nearly ...