That is correct.
You can only leave positive feedback for a buyer, or no feedback.
If the buyer agreed to buy an item and didn't pay, you should have waited 4 days then cancelled using 'buyer did not pay'.
The buyer would get a non-payment strike on their account which would last a year. With two non-payment strikes, buyers are blocked from bidding by many sellers.
Leaving comments like this 'Not a serious buyer. Not a positive transaction ! No transaction took place.' on a green positive dot, breaks the Ebay feedback rules and can damage YOUR account.
"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)