Just around the corner from Diagon Alley lies a dark, twisting alleyway devoted to the Dark Arts: Knockturn Alley. It's a place that the Weasley children are not allowed in, and with good reason; not only are the wares sold their a bit dodgy, but the wizards hanging around behave fairly suspiciously as well. From the shops and the street vendors one can purchase, among other things, shrunken heads, poisonous candles, human-looking fingernails, and Flesh-Eating Slug Repellent (CS4).
Some way down Knockturn Alley at number 13B (JKR) lies Borgin & Burkes, the most notorious of the alley's shops, which specializes in "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 & 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).
Items listed were on sale at Borgin & Burkes unless otherwise noted.
- a "bloodstained pack of cards" (CS4)
- poisonous candles, sold in a shop several doors down from Borgin & Burkes (CS4)
- "evil-looking masks" (CS4)
- Flesh-Eating Slug Repellent, which Hagrid apparently bought somewhere in the alley (CS4)
- The Hand of Glory, which "gives light only to the holder" (CS4) - purchased by Draco Malfoy at some point before 1996 (HBP7) and used to help Death Eaters break into Hogwarts, spring 1997 (HBP29)
- hangman's rope (CS4)
- human bones (CS4)
- human fingernails, sold by a street vendor (CS4)
- Slytherin's locket, which Borgin & Burkes purchased from Merope Gaunt for ten galleons and later sold to Hepzibah Smith for far more (HBP13)
- a cursed opal necklace, priced at 1500 Galleons (HBP6), which killed nineteen Muggle owners (CS4) and was later purchased by Draco Malfoy for an attempt to kill Dumbledore. Katie Bell was injured instead (HBP12, HBP27)
- rusty, spiked instruments (CS4)
- shrunken heads, on sale in a window across from Borgin & Burkes (CS4), one of which might have been purchased by Crabbe in 1996 (HBP11)
- skulls (CS4), at least one of which costs sixteen galleons (HBP6)
- giant black spiders, in a shop window a couple of doors down from Borgin & Burkes (CS4)
- a "staring glass eye" (CS4)
- a Vanishing Cabinet, which unknown to anyone but Draco Malfoy (at the time, at least) linked directly to its partner in Hogwarts. Draco bought it in 1996 and used it to sneak Death Eaters into the school the following spring (HBP6, HBP27)