FBA sold same book to 9 different buyers

I'm not even sure how this happened, but FBA made 8 customers angry at us because of their glitch.

We sent in a single copy of a book, which sold within a few days. If you're on Seller Central, you already know that setting the quantity of a title is out of the seller's control. We tell them how many we're sending in, they count and confirm the quantity before activating your inventory . (You may have seen other sellers complain that this process sometimes takes days or weeks). But adjusting the quantity is out of the seller's control.

But for some reason, FBA would not zero out the quantity after the book sold. They kept relisting it! We tried several times to manually close the listing, after realizing that others were trying to buy it. But the "Seller Central" system wouldn't let us.

So after 5 weeks of this nonsense, 8 other buyers also tried to buy the same book! But there was no book - there was only one copy, and it had sold 4 1/2 weeks ago. But the customers didn't know this and don't understand FBA. So we got 5 or 6 angry emails, including one negative feedback, because those buyers thought that we were trying to pull a trick, or that it was our mistake.

This may sound like a silly question, but how the heck can we get a negative feedback for a non-sale of an item we didn't have in stock???