I'm not sure where you can sell, but I don't think will get a
lot back in return; how old is it? Maybe you try selling to sgf
tech forumites? 
As for HDD, if you have data you do not want to let
loose, then better not sell it as data can be retrievable even
after reformatting.
I sold my old CPU to a karung guni man by the way. A few bucks were
all I got. 
