Whilst you are doing an honourable thing, eBay cannot act on one person notifying them of a fake item being listed. This could be coming from a disgruntled buyer, or even from a competitor wishing to have that seller removed.
eBay act on cases, if enough buyers open return cases for ' item not as described' this will raise red flags to eBay, selling restrictions can be placed on the seller's account, items removed, and ultimately lifetime bans from selling.
What you can do , is to forward any infomation and your findings, to the actual Brand owners themselves, and they can deal with the matter, if they wish.
@c00per210