Every method is safe as long as you verify.
1)If buyer gives you cheque, tell him you will hand over the car on 2nd day after funds are credited in your account
2)If buyer gives you Pay order, tell him you will hand over the car on 2nd day after funds are credited in your account. If he insists, call up the PO issuing bank and confirm the PO number and funds in buyers account by telling them account number. Still insist on handing over the car next day after depositing PO.
3)If buyer gives you cash, DO NOT count it or accept it. Take him to your bank and deposit it in your account in front of your eyes
After receiving cheque/PO, put a cover on the car and tell buyer he can note mileage or even keep the key. Its basically an assurance that you wont use the car in these 2 days
Just use common sense and be vigilant and you should be fine. And yes NEVER TRUST ANYONE, even your close relatives. I've sold 50+ cars and never faced any problem. Genuine buyers are aware of banking issues and understand 1/2 days delays.