This happens all the time with used Xbox Ones - explanation (Off-Topic)

by Mid7night , Rocket BSCHSHCSHSHCCHGGH!!!!!!, Monday, December 08, 2014, 08:06 (3437 days ago) @ Schedonnardus

any gamertag can play content on the "home" console. on other consoles, you have to be signed in to LIVE play the content.

Worked great for Halo LANs. If you had two 360s, you could have DLC on the "home" box, then let a guest play on it, while you logged yourself in on the second box.

The problem for the previous owner is that he will have to be on LIVE to use his own content. If his internet is down, he won't be able to play campaign offline, b/c he won't be able to connect to the license server. (at least that's how it was on the 360). Even if you fix things on your end, he may never know to switch his box to home, so he will probably run into a DRM problem eventually. you should still fix it on your end, if you can.

http://support.xbox.com/en-US/xbox-one/games/my-home-xbox

Got it. Well, I'll do what I can on my end.

Thanks, everyone, for enlightening me on this. This seems excessively complicated (and buggy) for something as common as buying used consoles.