Depends on the seller, some seller may not be ready to sale on pay order and ask for cash, though pay order is the safest of all, but in recent days fake pay orders have made sellers to think twice.
Whenever I sale my car I ask the buyer to come with cash in my Branch of bank, the buyer deposit money in my account in front of me and then and there I give him the possession of the car and also stamp vendor is sitting right beside so handing/taking over of car and docs is also done then and there, that's how I do. So again it depends on how seller wants things to be, and upon mutual consideration the deal is done.