Why does the old man begin by referring to the "spot remover"--its cost, and the need one might have to save up for it?

The old man in the story is in a criminal business and has to keep a low profile. This explains why he has such a nondescript and out-of-the-way establishment and why customers can only come to him by referral from previous customers.

An old man sat in the rocking-chair, reading a newspaper. Alan, without a word, handed him the card he had been given. "Sit down, Mr. Austen," said the old man very politely. "I am glad to make your acquaintance."

The old man is too clever to refer to the main product he sells--a poison--by its true name. It is especially desirable because it is undetectable. He also refers to it as a "glove-cleaner" and a "life-cleaner." These terms are somewhat appropriate because the person using the poison might want to wear gloves so as not to leave any incriminating fingerprints, and that person might want to start a new life by cleaning up his present one, so to speak.

The old man warns Alan Austen that the "antidote" or "life-cleaner" is very expensive.

"Do you know the price of this? For one teaspoonful, which is sufficient, I ask five thousand dollars. Never less. Not a penny less."

Austen is bright enough to understand the old man's innuendo, but he believes he will never have any need for such a substance. He is so much in love with Diana that he thinks it will last forever. The whole point of John Collier's cynical story is that love does not last forever. That is why the old man can make a good living by selling love potions for only a dollar. He knows the buyer will be back in time to buy the "chaser," which sells for five thousand dollars per teaspoon. The old man is trying to advise Alan to save up for the "chaser" because it is so expensive. This is Collier's way of informing the reader that it is easy to fall in love but that it can be very costly and complicated to get out of a marital relationship if a person wants his or her freedom. 

The old man tells Alan what to expect once Diana has fallen madly in love with him as an effect of the love potion.

"She will want to know all you do," said the old man. "All that has happened to you during the day. Every word of it. She will want to know what you are thinking about, why you smile suddenly, why you are looking sad....How carefully she will look after you! She will never allow you to be tired, to sit in a draught, to neglect your food. If you are an hour late, she will be terrified. She will think you are killed, or that some siren has caught you."

This sounds very much like a description of Mary Maloney in Roald Dahl's story "Lamb to the Slaughter," and it explains why her husband Patrick tells her he wants a divorce. Too much love and attention can be suffocating. Patrick resorts to the drastic expedient of telling Mary he wants a divorce--but she resorts to an even more drastic expedient of hitting him over the head with a frozen leg of lamb. 

Are John Collier and Roald Dahl offering a true picture of marriage? Maybe not, but something like forty to sixty percent of American marriages end in divorce.