If there was a request cancel transaction option from buyer, without having to ask seller directly, you would see an enormous increase in requests to cancel. Not good for sellers nor Ebay. Having to ask seller to initiate will make many think twice about it first.

In 11 years i have never had a buyer decline after requesting. I have had many not respond and so had to wait it out. Being all BINs loss of listing fee is insignificant so overall inconvenience to me to cancel is minimal. Auctions would be a different story due to loss of listing fee and potential underbidder.