When you have the traffic volume that eBay has, you can pretty much make your own rules without concern, despite what people say. And of course, traffic flow such as eBay has brings sellers, sellers bring traffic and the cycle of e-commerce just keeps going round and around. No one actually enjoys seeing so much of their margin disappear into eBay's pockets, but that's life.
There are plenty of other online platforms that are far more cost effective to market through, but hardly anything that provides the traffic flow of an eBay and traffic flow equals the opportunity of sales flow, what anyone online seller needs to be successful ultimately.