Does anyone use the marketplace to sell old books, dvds etc?

I listed two books a while back. they both sold immediately. Right after I got the sale notification, I received another notification from Amazon that these books were "out of stock" and I would have to change my listing. It was a bit confusing, but I did it. I never received any payment for the books.

Then about two weeks later, I get notice that the books sold again and another notice about the out of stock issue. I looked into it and saw that I had to provide shipping confirmation before I got paid and that each book had two separate buyers. To be fair, I shipped the books to the first buyers and cancelled the second transactions because of "lack of inventory" from their drop down menu.

I got a pretty nasty note from one of the buyers saying she wished she could leave me negative feedback blah blah blah and I never heard from the other one.

Now today I get an e-mail from Amazon saying that there was a case opened against me by the purchaser or oneof the books.

I am so confused. I sell tons of stuff on ebay and it is so easy to navigate and communicate with buyers/sellers, but with Amazon, I'm a little lost. I'm not sure if this woman who opened the claim is the one I shipped the book to or the one I closed because of lack of inventory or if somehow I didn't close that particular listing properly.

Either way, I shipped two books, never got paid and now have a claim against me.

Anyone know what's a seller to do in this situation? If you're an Amazon Seller, help!