they sell damaged goods and they do not want to take account for it

I have purchased a car early August at MCCarthy Menlyn, the car got stalk on the second day after purchase, i phoned Mccarty and told them but they just changed the battery, so after a week, i then realized that this are not the mileages that we agreed on with the sales person i , told him, and he started ignoring my mails and phone calls, then i drove to Mccarthy because i wanted to take there car back, the GM was closed, i then called the call center and they started taking me back to forth, telling me that i should trade in the car, how do i trade in the car t]within six months of purchase? i want mcCarthy to take there car , i do not want it anymore, please help me , i would really appreciate the help, because now the more i keep the car the more it will cause problems for me,