I have thought about this myself and the biggest issue that keeps coming back to me is privacy and who you can trust? If your to have any kind of transaction dealing with money you have to either give out your real name, real address, paypal email - which can disclose your name and address or you have to run on pure 100% trust that if you send someone a blank money order or cash that you get your items sent to a PO box.
If anyone knows anyway to do a complete private transaction, then I have plenty to sell!!!!
peace out,
daddy