I thunk they themselves face a bigger problem than you do - people calling them all the time asking about a car which has already been sold.
Though you're right, they should ethically remove their ads after the cars get sold, but if the don't, how do you expect pakwheels administration to call them up and ask each one of them if their car is sold or not?
The only possible solution I see is setting the validity of each ad to be one week. After that the person should refresh his ad if his car isn't sold yet.