What is the seller's return policy as stated on the listing?
If the listing says he accepts returns, then message him through eBay, tell him that you want to return the fragrance for a refund of the entire original price and explain the things that cause you to think it is counterfeit. Be polite, don't blame him (he may not have known he was selling a fake) and do not mention feedback. Keep it friendly. EBay policy is that buyers pay for return shipping, but you can request it. However, he will lose money when he returns your original shipping, so he may decline a request to also pay for your return shipping.
If he okays a return, send it back with delivery confirmation. This is essential; if PayPal can't see online DC, you can lose your ability to get a refund.
If his listing says "no returns", ask anyway. If he refuses, file a SNAD dispute with eBay and request a prepaid courtesy return label. Escalate it within the required time frame. After that, only communicate with the seller through the dispute console. Don't return it until eBay tells you to (in a written eBay message, not on the phone.) Then when prompted by eBay, return it with delivery confirmation. EBay may not require you to return it since the basis of your dispute is that it is counterfeit.
Disputes can be filed through eBay or through PayPal. Because this is a counterfeit, I suggest you file through eBay. They are the ones that can send you a prepaid return label or tell you not to return it, while PayPal is more likely to tell you to return it at your expense. (Based on what I've read on eBay boards.) Yes, eBay owns PayPal, but it seems some procedures are handled differently.
Don't leave feedback until the entire situation is resolved.