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

by GrizzNKev, Down the street from Microsoft, Sunday, December 07, 2014, 22:32 (3438 days ago) @ Mid7night

He can, but only on his gamertag. The way it works is that any account can play the console owner's games on that console, assuming the owner has it set as their home console. The console owner can also sign in on any other console to play their own games, or that console owner's games.