Ernest Hemingway Urban Legend

Over the past couple of years online, I've been seeing a six word short story ("For sale: baby shoes, never worn.") being claimed to have been written by Ernest Hemingway. The story is usually cited as an excellent example of flash fiction. You can see an example of this in an article about very short stories published by Wired. The story is often accompanied with a longer story explaining that the six word masterpiece was created by Hemingway to win a bar bet that he couldn't write a complete short story in just six words. Both stories are great, but they both smell of urban legend. I couldn't find the story in my copy of The Complete Short Stories of Ernest Hemingway, and having read a lot of Hemingway, I couldn't recall ever hearing about this story before. I checked with two experts, one on Hemingway, and one on misattributed quotations (Robert Trogdon and Ralph Keyes respectively), and neither knew of any source for the Hemingway story beyond the only legitimate source I could find. That source is Arthur C. Clarke, who in an 1980s essay on Reader's Digest called "The Power Of Compression" relates the anecdote of how Hemingway created the story (you can find it on page 354 of his essay collection Greetings, Carbon-Based Bipeds!: Collected Essays, 1934-1998). Clarke doesn't cite a source himself, and unfortunately he died earlier this year so we can't ask him where he heard the story. It's possible he heard it from Hemingway himself (though I haven't been able to determine if they ever met), or through writer circles, but since Clarke is rarely cited as a source for the story, much less where he got it from, I think the legend of Hemingway's composition of the story is probably a case of people seeing something on the Internet and then perpetuating it whether it's true or not. But, if anyone has more information on the source of this Hemingway legend, please pass it along. I suppose we should be thankful no one claimed the short story was written by William Faulkner, though that would be more amusing, since I don't know that dear old Faulkner could compose even a grocery list as brief as six words, much less an entire story.