There is no one definitive answer to this question, as people's needs and preferences will vary. However, some general tips that may be helpful include:-Checking to see if the phone is authentic and/or has been tampered with-Checking to see if the phone is unlocked and/or has a valid warranty-Checking to see if the phone is in good condition and has not been used recently-Checking to see if the phone is compatible with any of your service providers