Regarding the issue with regard to the item being sold, I agree we need to notify the seller slightly differently on the status of the payment, but when the user clicks buy the system needs to take the item off sale, otherwise you could end up with several users buying the same item.
We create the notification of the sale the moment the listing completes, for example when the auction finishes or someone clicks buy for a buy now product. To do it any other way would mean a seller might not be aware that his listing has ended or that someone has purchased stock.
Does that make sense?