A lot of book sellers are either charities or bulk dealers that get breaks on both shipping and fulfillment. For example, charities get their stock donated to them, and get huge breaks on shipping and ebay FVF. So they can charge less for their items and still can make money hand over fist selling books like that. These places have scaled it up to industrial levels too, probably far beyond what anyone would imagine they would do with the breaks they got them.
Like you say, anyone that doesn't have those advantages really can't compete. For instance, I have a couple of books I could never sell on ebay simply because it costs more to ship them out than a lot of these sellers are charging for the item. Then to get any possibility where I'm not paying to give away the product, I'm still making the equivalent of chicken feed. So it suppresses the market, and eliminates the possibility of anyone else selling their books. Product dumping is a huge issue in many categories on ebay, for sure!
