It's possible. I saw that Neil Gaiman's Fragile Things collection had been broken up into a bunch of $0.99 parts on the iPad store (though, oddly, there's no way to tell which short story is which without grabbing the sample). Quickly looking around I don't see anything else that does it, but there's not a good way to search by price on the iPad and I can't find a way to get into the iBooks store by iTunes. Which is also odd. (Yes, I got one. Employer mixed up which state income taxes I was paying, withheld at wrong rate, luckily in my favour. Also birthday.)
I'd venture that there would still be a need for editing and stamping approval. Quick excerpts of short stories don't work that well, and though $0.99 is a low barrier to entry I'm not sure I'd trust The Internet's ratings of a story, especially as there's likely to just be some huge mass of them out there and it'll take forever for a story to pick up enough ratings for the rank to achieve some measure of utility.