Pregnant and desperate for money, she has no idea what it’s worth and is only given ten Galleons for it. The locket is later sold to Hepzibah Smith.
According to Caractacus Burke, this happened "just before Christmas."
Merope Gaunt sells Slytherin’s locket to Borgin and Burkes
Date December, 1926
Location in Canon HBP10: The House of Gaunt
Type of Event Wizarding world