Some things on ebay can be sent off by the seller for authentification before they are forwarded to the buyer but I think that would probably be mentioned in the ad.
All the rest, you rely on the honesty of the seller so always check out feedback.
How do you expect ebay to know if a seller is selling fakes? You expect them to take just your word for it? What if you are wrong? What if you are just malicious or just after a refund? I am not saying you are, I am just pointing out there would be pitfalls for ebay if they banned ads or sellers just on one random person's say so.
You do have recourse though to a refund. Go open an ebay claim for item not as described or problem with the item. I wouldn't necessarily claim they are fake-mention whatever the problem is that has led you to class them as fake. Was it faulty stitching or sole, whatever.
You need to open a claim within 30 days of receiving the item.
