Euan, you’ve been misinformed.
Monroe BKA Saki did not write this. It is attributed to Hemingway, although there is no proof of that claim. Even worse, you’ve misquoted. It is properly known as
For sale: baby shoes, never worn.
Thirdly, this was no “short story competition” or sanctioned event. It was alleged to be him making a bet with his Algonquin Round Table buddies and trying to show off.