The eBay cancel button worked immediately for me. I saw the refund notice via email before seller even let me know he refunded me.
I thought it was weird the seller had 3 days to decide on whether to accept my cancellation request implying there was no guarantee they would.
As a buyer it is nice knowing eBay will almost always take your side of things. Haven't had much trouble there over the years outside of sellers not shipping items won at a price they aren't happy with. To have to wait over a week to get your money refunded in those situations is disappointing.
Over the years sellers have become more and more erratic. Used to almost count the item you purchased as guaranteed to arrive but now combined with USPS it's not the sure thing it used to be.
Mercari is even worse.