Scamming you on eBay for the value of an iPod is probably a misdemeanor. Stealing mail carries a prison sentence of up to 5 years. You decide which one someone is more likely to risk.
Get eBay involved and find your shipping receipt. It will show the weight of the package at the time of shipment. An empty box would weigh a lot less than an iPod. This is probably your only evidence.
In the future, I think you should use signature confirmation for anything you sell online unless you're willing to offer a no-questions-asked refund on the item.
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Some things are meant together, some things are better apart
Some things are easy, when other times they are hard
But that doesn’t mean what’s hard isn’t what’s meant to be
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Al Lewis