It means someone obtained a replacement for that device but has managed to keep it, and is now selling it. I would avoid it. And, be sure that the Apple ID Activation Lock has been disabled on any used device you decide to buy.
Activation Lock Problem
It requires the Apple ID and password of the legal owner. If you don't have it and cannot get it, then the device is useless to you. Return it for a refund, if you can, or take it to an Apple store for an exchange provided they will help you.