I just bought a domain name because I don't want to be at the mercy of ebay, which is currently my only location.
I agree with you that ebay has a huge advantage in being able to offer a combined shipping discount. I shop in volume too, and I offer cheap add-on shipping--under a dollar. But my cost to send the first book is not cheap because I use new materials--if you're doing volume, you get to a point where it's no longer practical to cut up grocery bags.
My biggest complaint is the new flat rate book pricing--which is posted, contrary to what the article says. First, the maximum of $4 is less than my cost--and I sell normal books. Rare books that need special handling and heavy books have to be listed with a shipping calculator. Second, it's not well thought out. 50 paperbacks have the same shipping maximum as 50 hardcovers. Hello?