If you haven't seen it already, the Guardian recently ran an overview of the online used book market. The article refers to the big A's (Abebooks, Alibris, Amazon.com), and interviews two British online booksellers. Not particularly interesting, except as an indicator of general press interest in the field.
eBay's been encouraging Half.com booksellers to move over to the eBay platform, but many booksellers have been checking out some of the other options. AuctionBytes is running a three-part series called "A Look at Half," about how Half.com sellers are dealing with the site's closure.