To clarify, you will need a unique game registration code in order to play the game, and used copies will not be eligible to receive a key if the code has already been redeemed.
http://forums.elderscrollsonline.com/en/discussion/comment/1801851/#Comment_1801851
Not really a surprise considering it's an MMO, although I think it may be news for some console Elder Scrolls players as they're used to the game media being the game media, not the box basically being a license.
UPDATE -- U-Turn! - Bethesda have clarified Zenimax's original statement was incorrect - the game does NOT need a key to use and the media can be resold. ""The physical console version of The Elder Scrolls Online: Tamriel Unlimited does not require a one-time game code, as with any other boxed PS4 / Xbox One game only the game disc is needed for verification," Bethesda said in a statement this afternoon. "Any previous statements counter to this were incorrect."