As well as many shops, Diagon Alley also contains small stalls. These stalls sell a wide range of things; including magical sweets. They set up stalls selling amulets and other objects, which (according to them) protect you against werewolves, Dementors and Inferi. These "dark magic protection" stalls, however, are illegal, and likely scams. Arthur Weasley is the one in charge of arresting their owners.