Counterfeit item

I ordered an item on Vinted which, when I received it, was clearly counterfeit. I raised the "seems counterfeit" issue and the seller has come back with a story about having two of the same thing and mixing them up, and offered a minimal refund. I've said we'd want a full refund for this and an awaiting an answer but have some questions in the meantime - is the seller able to require me to return the item in order to get the refund? I don't want to keep it, but I imagine if I return it they will then just try the same thing on someone else? Are there no other consequences of selling counterfeit items on Vinted? Also, if I return it who covers the cost of postage?