You'll probably want your money back, so contact the seller and politely tell him how you know the item is a counterfeit (he might have been suckered by his supplier, too).
If he asks you to return the item for a refund, politely again, inform him that UK and EU legislation prohibits the return of counterfeit goods to the supplier. [The relevant law in the UK is section 92 of the Trade Marks Act 1994.]
Inform Trading Standards in the seller's area of the transaction and offer to send them the item for confirmation of its counterfeit status and its authorised destruction. They're busy people and not too interested in the small fry so they may not need it and prefer to have a stern word with the seller.
If necessary, open a dispute case against the seller arguing against the item's return on legal grounds and stick to your guns (eBay seems to make no distinction between SNAD and counterfeit items).
After 10 days you can escalate the case if you and the seller can't agree.
eBay may still adjudicate that the item should be returned but once you point out that in doing so they are leaving themselves vulnerable to a charge of accessory to a crime you should receive your full refund without having to return the item.
You now have a free counterfeit item so what to do with it?
If you remove the brand names and logos it is no longer a counterfeit but a simple fake.
Some items like USB drives can be "unfaked" (ie restored to their true capacity) but beware, the process might be reversible so selling the item could cause you problems.
But at least you have something that may be of use to you and the seller has learned a valuable lesson about dealing in dodgy goods.
My recent experiences have highlighted a few new developments.
Firstly, the counterfeiters have shifted their strategies in selling flash memory devices.
They used to sell low capacity devices 'spoofed to appear as high capacity.
Since so many people caught them out with software such as H2TestW, they've now switched to selling full capacity devices labelled as higher speed memories (e.g. a compact flash card rated at 30Mb/s labelled as 100Mb/s).
Although H2TestW gives Read/Write speed readings they are not as accurate or thorough as those provided by ATTOtest Bench32 freeware.
Secondly, eBay are inconsistent in their adjudications on counterfeits, demanding the item's return before a refund is issued unless you can provide 'acceptable proof' from the brand owner.
eBay define 'acceptable proof' as
"This document should be on letter headed paper and contain the name, address and the telephone number of the document author." and
"All documents sent to eBay must Comply with These Guidelines: .
1 The only supported image types Are: JPG , GIF, PNG, BMP, TIFF
2 The only supported File types:..... PDF, doc, docx, rtf, txt .
3 We Will only Accept Files up to 8MB"
So it pays to get all your ducks in a row before opening a claim.
It might also be prudent to ignore the eBay Resolution Centre altogether and claim through PayPal or your bank account/credit card company.