According to CarsDirect and DigitalTrends, the top major used car sales websites are Cars.com, AutoTrader and eBay Motors. A third party review site should be consulted for a full listing and comprehensive comparison of car sales sites.
There are several different sources for used car sites online. Some are independent or owned by non-automotive companies like eBay Motors or Cars.com, some are run by large dealer groups like CarMax or AutoNation, and some are operated by automotive sales and service companies such as NADA or Kelley Blue Book. There are strengths and weaknesses at each site, so it is advisable to search several sites and understand what each offers and restricts from its listings of cars available for sale.