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.
In Scandinavia I think Tise is similar. For purchasing used clothing and stuff, but the listing is more akin to Instagram than on a normal listing site. You follow people with a style you like and buy their stuff, and then sell it on.
It's interesting that they have a much higher valuation (and from some cursory research, moderately more users) than Depop, despite (AFAIK) being completely obscure in the US/UK.
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/