Borgin & Burkes

Some way down Knockturn Alley at number 13B, lies Borgin & Burkes; the most notorious of the alley's shops, which specializes in "objects with unusual and powerful properties", and particularly items that are both "unusual and ancient". The shop is quite large, dusty, and dimly lit, with a large stone fireplace, and is known to be frequented by families like the Malfoys. 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.

This is not a place usually frequented by Hogwarts students, although there are exceptions to any rule. Many of the items in the shop are dangerous and probably well best left alone.