If the buyers open a case stating the item has not been received, but you have proof of delivery, then the cases will close in your favour if you added the tracking when you posted the items.
Block the buyers concerned.
Edit
The reason you need to choose if you report the buyers is 'abuse of the Money Back Guarantee'.
You can report an issue with a buyer by using the below button.
Report a buyer
"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)
