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.