I've never traded phones, but the same logic I do when selling/buying mine still likely applies. Make sure you do the trade at the carrier store. That way you can verify their phone has a clean IMEI and isn't on some payment plan. If it's on a payment plan and they stop paying it after doing the trade, then that phone will get blacklisted. Also, just test out as much as you can to make sure it isn't damaged. It may look perfect on the outside, but who knows how the internals are especially if it ever got in contact with water. Play some audio, test out the mics (phone call, recording, etc), and shoot some pics.