Depop isn't alone in this, Vinted[1] has a current valuation of $4.5B and is hugely popular in Belgium, Netherlands and France (maybe more places, but those i can definitely say).

In both platforms there is influencer-heavy marketing. Many fashion influencers(i really dislike this term, but it is the nomenclature) sell clothes that they wore very little in Instagram advertisements. This is a great business model for them, people follow them more tightly because they are actually able to buy for (mostly) affordable prices some of the clothes they see in pictures and they also get some additional under-the-radar cash.

[1] : https://www.vinted.com/