eBay cannot act on a message from a member, as they do not see the item physically themselves, and the report can, and does sometimes come from a competitor or even a disgruntled buyer.
eBay work on cases. Should an above average number of cases be opened showing a seller is selling items not as described ( fakes), this raises red flags to eBay, and then they can pull the items and even ban the seller from their site.
You could for a quicker action, if you like, contact the actual manufacturer of the item if you spot a seller selling their Brand items , in counterfeit form. Up to them to contact eBay, any instructions from the Brand owners to pull items, will be acted on.
@stuffbyiris