Surely if the item is counterfeit you would not be happy with a partial refund, but would wish to return it for a full refund, which you are entitled to under eBay's 30 day MBG. I also doubt the seller would agree to a partial but would tell you to send the item back for that full refund.
What does their feedback say? Are there any disgruntled buyers showing items bought were not as described ?
Where is the seller registered UK or China?
@alisonlwood2012