Make sure you message the buyer and ask them if they are sure that is the item they got from you and not another seller.
Point out it is not the item you sold them or sent and you will have to ask the carrier to investigate what happened.
If they don't back down, and they send you the wrong item back, report them to action fraud and get a case number.
Then, you need to report them to Ebay and request Ebay to look at the case.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
