"For sale, baby's shoes, never worn".
But is it actually a story? For the reasons below I would say not. It is definitely a narrative, because it has a movement forward; From the neutral commercial opening "For sale", it has moved on by the end to deliver a powerful emotional punch, as the implication is that the shoes were never worn because the baby has died. But the devil is in the detail and 6 words just cannot provide any such detail. There are too many unanswered questions for the reader to decide that this is anything more than a taste of a story rather than a complete one.
1) Baby's shoes: newborns don't really wear shoes. They are in babygrows with pouches for their feet. So it's unclear whether the baby died during childbirth or in the home. Since babies don't walk, while in the home they are unlikely to wear shoes, though they may have them for when being taken out in a pushchair or sling, where the shoes are purely decorative adornment. The shoes don't have to provide any support for the foot, since again the baby is not putting any downward pressure on the foot against the ground. Therefore such shoes are flimsy, unless the parent has indulged in a designer brand trainer/sneaker (though presumably Hemingway was writing before the ubiquity of the branded sneaker took hold). They are 'booties' rather than shoes perhaps. So the question has to be asked, why bother putting them up for sale? They are low-cost items, except perhaps if a brand trainer/sneaker is the item in question. What resale value could such short-lived accessories have?
2) Why is a grieving parent putting baby shoes up for sale anyway? If it is to banish a painful reminder from sight, just throw or give them away or burn them? The emotional impact of the tragedy in this tale is a tad compromised, if a parent has the wherewithal to put up an advert to sell such a low-cost item. One would certainly like greater insights into the parent's motivation behind such an action. This part of the story is too incomplete.
3) Time is always an important quality in fiction. There is no way of knowing how long after the baby's death the advert was put up. Is it at the end of a grieving process of some length, therefore possibly representing closure? Or is it as above still in the rawness of the immediate aftermath? The narrative moves, but we are uncertain along what timescale, the opposite ends of the possible spectrum again inflecting the emotional state of the character very differently. Never worn suggests that it is either in childbirth or so soon after as the baby never taking an outing in an upright pushchair or a sling so as to require decorative footwear. But it is not conclusive.
It may seem churlish to spend so many words to critique a 6 word piece, but I wanted to use it to illustrate some of the differences between narrative and story.