The owners of half.com (ebay) closed the service on the first of September.
(They have a FAQ
HERE.)
I know about the listings on abebooks.com, alibris.com and biblio.com. What other sites list books sold by other vendors? Do any still allow individuals to sell books on the site (besides ebay)?
bookfinder.com does list sites where books can be found, but it is not a bookseller.