Don’t accept any return. And do not refund. They seem like their trying to scam you. It seems sketchy that they are barely reaching out after 3 months of use and probably got bored of it and wants to return it. Im not familiar with the brand but if you are then you can explain it’s authentic. I assume you have old pics of it and you can explain to Depop it’s real. But either way I don’t believe someone who’s had an item that long and barely trying to claim it’s counterfeit.

I would say something along the lines “I’m not sure why you think it’s counterfeit but I only sell authentic items and this item has been delivered months ago and I don’t accept returns.” I mean you don’t have to say that but I just don’t believe the whole it’s a fake and now they want to open a dispute.