I think people are avoiding the earlier model S' due to a lot of issues they had. Poor(er) fit and finish issues, some huge oversights that lead to corrosion of the body, center touchscreen issues, etc... I think a lot of people are just nervous about buying a car from the 2012 and 2013 era because of that.
Tesla can sell them for more, because they're selling to people that are new to Tesla entirely. Those of us on these forums, however, know both the good and the bad about Tesla.
^ Just my two cents.