"The Chaser," like many of John Collier's stories, is an example of metafiction (see the reference link below). The story is not intended to be taken too seriously, although the underlying message of the story is a very serious one. Everybody knows there is no such thing as a love potion, especially one as effective as the potion the old shopkeeper is selling. The magical elements are essential to the story without being at all credible. That is why so much of the action takes place offstage, so to speak. We only know from the old man's dialogue what is supposed to happen when Austen slips the love potion into Diana's drink and how Austen is expected to feel in the future when he comes back to the mysterious little shop for another potion that will enable him to be rid of Diana without getting convicted for murder.
If the reader accepts, only hypothetically, the idea that there is a shop right in the modern city of New York that sells love potions and undetectable poisons called "glove cleaners" and "live cleaners," then the reader will probably accept the old man's predictions that the potion will work only too well and that Austen will want the "chaser" to free himself from her suffocating possessiveness.
Collier preserves an Aristotelian unity of time, place, and event by confining his story to a single setting, the interior of a little shop. We know from Austen what has happened in the past and from the shopkeeper what will happen in the future. One question which seems to remain unanswered is this: Would the love potion do its work and then lead to a normal love relationship, or would the cloying possessiveness described by the old man be the effect of his love potion? Would her love be normal or abnormal? The story seems to be suggesting that all love between men and women becomes suffocating after marriage.
The magical elements of Collier's story are the essential part of his "premise," and if the reader grants the author his premise, then the rest of the story will hold his interest--at least until he finishes reading it and has experienced the effect the author intended to achieve.