"She was gonna fine me, but she took a fancy to the locket an’ told me she’d take it and let me off that time an’ to fink meself lucky."
-- Mundungus Fletcher (DH11)
Mundungus Fletcher admits to having stripped 12 Grimmauld Place of its valuables sometime after Albus Dumbledore’s death. While trying to sell them in Diagon Alley, Mundungus was approached by Dolores Umbridge, who asked to see his Ministry sales permit. Instead of landing him with a (well-deserved) fine, Umbridge took the locket he was selling instead (DH11).
This would have taken place between Dumbledore's death at the end of the 1997 school year but before Harry returned to Grimmauld Place during what would have been his seventh year at Hogwarts.
That this important Horcrux artefact could be stolen and offered by the "low-life" Mundungus Fletcher for sale to any random wizard passing by is astonishing. Having split so many pieces of his soul and leaving them scattered around the Wizarding World, Lord Voldemort certainly failed to look after them closely or be able to "feel" when one of them was in the wrong hands.
From the Web
Harry Potter Wiki: Slytherin's Locket