From this page:
https://www.welivesecurity.com/2015/02/09/common-ebay-scams-avoid/
"eBay scam 4: The bait and switch
How it works:
You’re selling a common item – an iPhone, perhaps. A buyer wins your auction, pays up and you ship them. So far, so good. Then, however, they get in touch with photographs of a iPhone with a broken screen, claiming that you sold them a dud, or that they broke in transit. They complain to eBay, and with nothing but your word against theirs, eBay sides with the buyer (it’s called the Buyer Protection Policy) and forces you to issue them a refund. They’ve got your fully-functional ‘phones AND your cash.
How to avoid it:
Tough one – if you’ve been stung by it, it’s too late. The best defence is pre-emptive: make sure you ship valuable items with insurance, that the buyer pays for. It’s possible to list with ‘no refunds’ but may deter genuine customers."