How did the people of Marley steal 7 of the 9 original titans? The story of Attack on Titan says the Eldian Empire grew unstable due to internal conflicts between the families with the power of the Titans, and collapsed during the Great Titan War a century ago, and that Eldia lost 7 of the 9 Titans to the nation of Marley.
How did the people of Marley steal 7 of the 9 original titans?
Actually, it was 8
not 7.
As simple as I can say
when king fritz decided to go and live on the paradise island. He only took the founding titan with him. And the other 8 titans remained on the mainland.

7 of them in the hand of Marley and the other one (Warhammer titan) in the Tybur family.