Rowling's short story is the star lot at a charity auction at the flagship of Waterstone's book store chain in London. Proceeds will benefit the writers' association English PEN and a dyslexia charity.
Rowling was able to squeeze her Harry Potter prequel onto both sides of a piece of A5 paper, which is slightly bigger than a postcard.
"Given the enormous interest we have seen in recent times for autograph work by J.K. Rowling, the prospects for her story card are good to say the least," said Philip Errington of Sotheby's auction house, which is running the sale. "There is just no telling how high the bidding might go."
Organizers would not comment on the content of Rowling's card, but the prequel to the seven-book series does not offer hope for a new Potter novel. Rowling finished her card by writing, "From the prequel I am not working on - but that was fun!"
Twelve other authors and illustrators also contributed cards to the auction, including Nobel Prize winner Doris Lessing, playwright Tom Stoppard and novelists Nick Hornby and Margaret Atwood.
Lessing wrote about the power of reading, Stoppard contributed a short mystery and Hornby turned his card into a collage about a superhero.
Atwood will fill out her card remotely an hour before the auction using a touch pad in Paris linked to a robotic arm in the bookstore.
Copies of the cards will be on display in Waterstone's stores and online following the auction, and they will be collected into a book available in August.
The final installment of the seven-book series, "Harry Potter and the Deathly Hallows," was published last year.
Rowling has said she has no plans to write another novel about the boy wizard, but in December she sold a handwritten, leather-bound book of fairy tales she described as drawing on the series' themes for nearly 2 million pounds (US $4 million) at auction. The money went to The Children's Voice, a charity Rowling co-founded in 2005.