Wayne,
Most of these "too good to be true" deals are perpetuated when someone's account becomes hijacked. This can happen any number of ways, the owner of the account might have answered one of these spoof emails asking you to update or confirm your user account information, or he had his email address/account compromised (aol is the worst for this) and that gave the hijacker all the access to change passwords and such and take over the account. These spoof emails look so realistic that if you did not know better and did not pay attention to the headers when you clicked on them, that you would think it was Ebay or Paypal that had sent it. As an example, click the following Ebay link (safe) and see for yourself.
Cobra Sale / Strange Bird
See what I mean, the link says one thing, but takes you to a completely different web page.
Simple and effective for the scammers, they send out a million of these emails a week and wait for the 1-2% of the people who are not paying attention to respond to them. Scary but true.