You can cancel a sale if you don't want to go through with it for some reason. If the buyer has paid, this will have a negative impact on your account.
What is the background to this?
How has the buyer contacted you to 'claim' they have bought an item?
Why should you want to cancel it?
"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)