There have been a few threads on here recently about people who buy a product then raise a dispute in PayPal or a chargeback with their credit card provider just so they can steal your product.
There's another one to be wary of and that is people who pay using an eCheck/eCheque then cancel it before it clears.
If your ordering system is set to fulfil orders before an eCheck/echeque clears then you're open to having your product stolen unless you can disable it remotely.
And if you don't have the facility to disable your product remotely, you might want to consider not fulfiling the order until the eCheck/eCheque has cleared. This is what Paypal recommends anyway.