"Skulkin' around Knockturn Alley, I dunno dodgy place, Harry - don' want no one ter see yeh down there -"
-- Hagrid, upon finding Harry in Knockturn Alley (CS4)
Some way down Knockturn Alley at number 13B (JKR) lies Borgin and Burkes, the most notorious of the alley’s shops, which specializes in the buying and selling of “objects with unusual and powerful properties” (HBP20), and particularly items that are both “unusual and ancient” (JKR). The shop is quite large, dusty, and dimly lit, with a large stone fireplace (CS4), and is known to be frequented by families like the Malfoys (CS4, HBP6). Its proprietors, Mr. Borgin and Caractacus Burke, are smooth talkers known to buy magical objects for much less than they’re worth, and sell them for far more (CS4, HBP13).
When Tom Riddle graduated Hogwarts, he surprised his teachers by taking a job at Borgin and Burkes, though they gladly hired him as he was an enchanting young man and able to “persuade” owners of valuable magical objects to sell them to the shop. What his professors didn’t know was that Riddle wanted access to these powerful objects, already thinking of his Horcruxes. When Riddle went on one such visit, to the home of Hepzibah Smith, she showed him two valuable objects that she possessed: the cup of Hufflepuff and Slytherin’s locket. Upon seeing these, Riddle murdered Hepzibah, framed her house-elf, stole the items, and vanished (HBP13).
Harry has visited the shop twice; first as a twelve-year-old, when he accidentally took the Floo Network to the wrong destination, and watched Lucius Malfoy come in with Draco and sell items to the shop (CS4); and second four years later, when he secretly followed Draco to the shop and overheard him purchasing the vanishing cabinet and threatening Mr. Borgin, though Harry did not figure out what Malfoy had been buying until many months later (HBP6).