Final answer:
The one thing that no one else knows about the merchant is that he is listening eagerly to every chance word in the street, as if he never should hear human voices again. This detail helps the reader understand the merchant's desperation and fear of isolation.
Step-by-step explanation:
In the given excerpt, the detail that identifies the one thing that no one else knows about the merchant is that he is listening eagerly to every chance word in the street, as if he never should hear human voices again. The speaker throws this detail into the description to emphasize the merchant's desperation and fear of isolation. This detail, presented as an afterthought, contributes to the reader's understanding of the merchant by portraying him as someone who is deeply lonely and afraid of losing connection with others.