If the buyer just doesn't pay, you do have to wait 4 days and can then cancel using 'buyer didn't pay' so the buyer gets a non-payment strike.
However, if they ask to cancel and you don't respond to their request, Ebay will step in after 3 days.
Relevant rule is here:
https://www.ebay.co.uk/help/policies/member-behaviour-policies/order-cancellation-policy?id=5298&st=...
- 3 calendar days after the request date
If the buyer has paid for the order and the seller doesn't respond within this time frame, we'll close the request and the order won't be cancelled
If the buyer hasn't paid for the order and the seller doesn't respond within this time frame, we'll automatically cancel the order
"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)