I'm not sure what it means, I mean who actively wants a timewaster?
It screams of 'If you come and view my car/household item/whatever I'm selling and don't buy it, I will consider you to have wasted my time.'
After viewing something, especially a car, there's a million and one reasons why you may not choose to buy it. Not least because the item isn't quite as 'mint' or 'perfect' as the seller claims.
Sticking 'no timewasters' on an ad will almost inevitably make me skip the ad and look for something being sold be a person who isn't arrogant and hell bent on alienating potential customers.