This scam targets almost every seller on ebay. Basically the seller is tricked into believing payment has been made, he/she then sends the item, to only later find themself without their money.

Below are some of the most common non-payment techniques used on ebay:

• Fake Money/Money Order/Checks: The scammer sends the seller a fake money order, fake check, or counterfeit money.
  The seller sends the scammer the item before the payment has been successfully cashed.
  When the seller tries to cash the payment he/she finds out it is fake.

• Credit Card Chargeback: The scammer pays a seller for an item using his/her credit card (either directly or via paypal, bidpay, etc.).
  Once the scammer receives his/her item, he/she falsely initiates a chargeback.
  Unless the victim has enough evidence to prove that the item was sent and successfully delivered, the victim loses his/her item, and has to pay a large chargeback fee.
  
• Stolen Credit Card: The scammer pays with a stolen credit card. The victim sends the scammer the item. The owner of the stolen card reports the card as stolen and a chargeback is initiated. Seller loses his/her item and has to pay chargeback fees.
  
• Fake Payment Received Email: The scammer sends a phony email to the seller pretending to be an online payment notification.
  The notification says something like "Your scammer has just sent you payment for your item via (paypal, bidpay, ecsrow, billpoint, etc.)."
  The victim thinks they have payment and sends the scammer his/her item. Soon the victim finds out that the payment was never really sent.

Victims of non-payment are tricked into believing that they have payment for an item which in actuality they don't.
Sellers should always confirm not only that payment is received, but that it is valid before sending a buyer his/her items.