Marketplace sellers

I'm sick of people "selling" items in the Amazon marketplace that they don't actually have. Can't tell you how many times I've ordered an obscure cd or dvd from a marketplace seller who either have to try and scramble to get me the copy I paid for from another vendor or end up refunding my money. Amazon makes it clear that if you don't have an item you don't list it!! Why is this so difficult? The marketplace reliance on the cheapest, crappiest method of shipping - USPS Media Mail - is another sore spot. Buyers pay enough to cover 1st class shipping at the very least yet these cheapskate sellers pocket that money and ship it one step above pony express! (Ebay is equally guilty of both charges too)