You paint a pretty picture; the camaraderie of buying, selling, haggling in an online cobbled internet town square, make for an idyllic setting. The tug-and-pull of reuse/recycle, vintage vs fast fashion are noble principles.
However, there is a murky side, which is infused with extreme expectations, toxic and churlish behaviour e.g. Amazon Prime type service, petty haggling over a few pennies, sellers abusing their privileges, limited understanding of fees and/or how PayPal works, unnecessary spats and an appallingly high level of entitlement etc.
Nevertheless, there is synergy, but have doubts about long-term prospects ─ Depop might just become another case study in acquihire or perhaps just a blot, as there is no shortage of other contenders nipping at their heels e.g. Vinted.