Doesn't have to be a bit coin auction site though. Those are ebay fees and how they make their money even if they accepted BTC it would still be the same.
They're making way more cash than they need to. Paypal is owned by eBay now so basically they're double charging like greedy idiots. The sad thing is they're the only widely known site where you can sell things, basically what every business does when they start to get big. They have no loyalty to their customers.