It seems to me that Ebay should implement a setting in the process for listing coins, probably alongside the "condition" box where, for single coin listings, the seller has to click either "Genuine" or "Copy" - not able to proceed until one or other has been selected. Then, if they have clicked genuine and it is not, they are in clear breach of trading standards.
It takes away the opportunity for sellers to say nothing and allow buyers to assume it is genuine.