How did they pay?
If they paid through Ebay checkout, it will automatically be marked as paid. If they paid you cash, then there is not any way to mark as paid because cash on collection is no longer allowed.
"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)