I'm not going to be an apologist for Ebay because a lot of what they do (Paypal for instance) is shocking, but if you were to ask a Record Dealer to sell stuff on your behalf I'm sure they would charge a higher percentage than Ebay. In terms of what you get it's not actually that bad compared to the likes of GEMM say, who charge (I believe) 15% and offer less exposure, and a far less slick operation.
As a buyer, Ebay offers a pretty good user experience. As a seller, less so, but many would contend it's a necessary evil until something better comes along.