Yep been done on ebay, Bought some R/C gear paid by paypal. Item was sent registered with delivery confirmation. When i got to the PO to pick it up i thought the box felt lighter than expected. Got home and opened it to find half a housebrick and a heap of newspaper. Ebay/Paypal couldnt do a thing about it as the seller had proof that the package was delivered. As i had paid by Paypal using my creditcard, i went to my bank who refunded the money and went chasing the seller for fraud.
Always use Paypal and by Credit Card if poss
Get items sent Registered(free insurance to $100), I get mine sent Person to Person so only i can sign for it and Auspost will ask for ID to ensure it is you.
99% of buying through ebay will be good, there will always be people trying to rip unsuspecting buyers off