What to do to get refund and money back?

In short: Bought amd rx 6800, furmark shown 0 arfefacts, ok temps

After long negotiations, we agreed on the price, the card is delivered to me.

GPU gives visual artefacts with drivers, without it, it's the same AI says that probably VRAM is bad, and in my pc, furmark also shown 0 anomalies because it can't detect them

I want to return the card because it's not working, make photos of errors to justify, I wait few days to even be able to, vinted gives support very slow.

Card delivered to buyer, now he is doing the same, blocking refund because card shows artifacts, and now I wait 2nd day for vinted action

Do they support either buyer or seller?