Half.com is owned by EBay but they are not an auction site. They are a clearance
center for new and gently used media, mostly for textbooks. I used them heavily when I was finishing graduate school. Just go there and type in the title, author, or ISBN and they will show you everyone who has a copy for sale, then you deal directly with them through a link. It is rarely individuals. Usually used book stores. There is a feedback/star system to tell you how many sales they've made and how reliable they are.