There's no way to stop them completely.
The best you can do is be safe as you can and follow the advice Skyline gave when trading.
But the charge back of 'Unverified Transaction' can't really be avoided under any circumstances.
So just make sure you do a great check on whoever you are selling with. Also another good thing to do is limit how much you sell to each person.
PA works as a middle man, the money goes through them before it reaches the other party so no chargebacks can happen since you are essentially getting the money from PA. Only problem is that they charge $2 + %10 to gold sellers :/