Why do they mention the poison in the story?

In the story, poison is mentioned as a "life-cleaner," an euphemistic description for murder.
Alan Austen, the protagonist, anticipates that the love potion will win him the woman of his dreams. In purchasing the love potion, Alan is motivated more by selfish indulgence than love. The text tells us that he is fascinated with Diana, a woman who is at best indifferent to his attentions. For his part, Alan is so desperately in love that he is willing to use deceptive means to achieve his romantic goals.
Certainly, the love potion's apparent potency sounds too good to be true even to the lovesick Alan. However, he is willing to suspend disbelief in order to have Diana obsessed with him. In Alan's mind, he is entitled to Diana's love and cannot conceive of her rejection. Indeed, he will not accept her indifference with any sort of grace. It never occurs to Alan that one cannot force love or rely on unnatural means to create attraction.
The author implies that there are many "Alans" in the world who will have later need of the poison or "life-cleaner" that the old man sells. The poison is mentioned to make a point: love that is won by deceptive means fails to bring true satisfaction. In the end, Diana's cloying love feels smothering to Alan, and he resorts to equally desperate means to rid himself of her.

Poison is what the old man sells to customers who have bought his love potion. He knows that, one day, the men who buy his potion for a song will come back for a drop of the harder, more expensive stuff. After years of ceaseless love and devotion, they will feel stifled and hemmed-in by the needy wife or girlfriend intoxicated by the potion's magical properties. Though at first they think such endless devotion is completely wonderful and the answer to all their selfish prayers, these men eventually end up totally bored, trapped, disillusioned, and desperate. The old man, like the author, has a rather cynical view of love. He seems to think it is some kind of deadly disease, whose inevitable symptoms can only be "cured" by the lethal administration of poison.