I would also recommend swappa, having bought a used OneX there when mine went into the lake with me.
Factory unlock means you can use it on any carrier it is compatible with, whereas phones you buy from ATT, you cannot take to Tmobile as it is "locked" to ATT. You can get them unlocked within certain parameters, but it takes effort. If you are on ATT, get the unlocked or ATT version. If you have a choice between either for same price point, get the unlocked as you could later sell it easier.
EDIT- If you are a normal user, get a non-rooted, stock phone. If they all seem rooted, ask the seller to restore it to stock... it can be done, and they should have the know how.