As noted, in the US and UK, above a certain value, eBay requires authentication. In the US, it is also optional (with a fee) for lower value watches. Frankly, while this thread shows that it can be problematic for the seller on a rare occasion, I think it is overwhelmingly to the seller's benefit. Once a watch has gone through authentication, it is extremely difficult for a buyer to play games, tamper with the watch, or try to return it based on false information.