The entire story takes place in a single room. It might seem that in this particular story John Collier is trying to adhere to the Aristotelian unities of time, place, and action. The time is the present. The future is only alluded to by the old man who sells magic potions. Collier describes the setting in the opening lines.
Alan Austen, as nervous as a kitten, went up certain dark and creaky stairs in the neighborhood of Pell Street, and peered about for a long time on the dim landing before he found the name he wanted written obscurely on one of the doors.
He pushed open this door, as he had been told to do, and found himself in a tiny room, which contained no furniture but a plain kitchen table, a rocking-chair, and an ordinary chair. On one of the dirty buff-colored walls were a couple of shelves, containing in all perhaps a dozen bottles and jars.
The setting strongly suggests that the proprietor is keeping a very low profile because he is dealing in illegal drugs and probably doesn't have a business license. This old man doesn't seem to care about attracting customers. He gets all his business through word of mouth. We can see that Alan Austen has gotten the name and address in confidence from some friend. He has been told to go directly inside without knocking. This is probably because the old man doesn't want his neighbors to get any notions about how many callers he has. In fact, nobody in this seedy neighborhood knows that he is selling anything. He is just an old man living alone in a cheap little furnished room. But we can surmise that he is only here during certain hours and probably has a luxurious residence elsewhere in the city (which could be either New York or London, but probably New York, since the story was originally published in The New Yorker in 1940).
Alan, without a word, handed him the card he had been given.
This is a shady business. We learn that the old man is making all his money from selling poisons which cannot be detected in autopsies. There is hardly any evidence that he might be running an illegal operation. The bottles and jars on the shelves could contain practically anything and are probably just there for show. None of these bottles and jars may contain either the love potion or the poison. When Alan asks about the love potion at the end:
"Oh, that," said the old man, opening the drawer in the kitchen table, and taking out a tiny, rather dirty-looking phial. "That is just a dollar."
He keeps some of his real merchandise out of sight. The fact that he calls the poisonous "chaser" a "glove cleaner" underlines the secrecy of his business, which further explains the dilapidated setting.