You need to track the shipment to the seller and request signature for item value >$250. Once it's received (s)he should refund your $. If (s)he does not, escalate to eBay and they will force a refund back to you. Leave appropriate (negative) feedback describing your experience without resorting to name calling or coming off as stupid.
Seller's options for making offer are to offer a partial refund, complete refund on return of the item, or for you to just keep it.
You do not deal directly with the seller - you dispute this through Ebay. Ebay's rules show that you must provide proof of delivery back to the seller before your money is refunded. You are required to pay return shipping if you file a dispute
Tell them that they refund your money right away, and once you get your money you will send it back. Or you could contact ebay and tell them what has happened.
how do you know its counterfeit?
you don't tell us
I would love advice from someone who has had this happen before. I purchased a bottle of perfume on ebay, and it is counterfeit. I've made a complaint and the seller has sent me an offer for me to mail it to an address and "when they receive it" they will refund me. I don't like this for several reasons: I don't want to pay shipping, they don't need this back so they can resell it to someone else- it is NOT advertised as a fake, and how can I trust them to give me my money back? What should I say back?