The feedback system is a great way to validate a user's credibility but it is not fool proof. The only way to be entirely safe is to use a 3rd party trade option (a moderator is the middle man).
Your best bet is to ensure you have exchanged all contact information, including: name, address and phone number. It's a great idea to have a phone call to finalize all details and to be sure the number is working.
Be very clear about the details of the trade: condition, what comes with it, etc.
Communication is KEY.