I've had two cancellations in the last few weeks - one took TWO DAYS, the other cancelled in 12 hours. These were both buyer requested. I have no idea why these cancellations take so long to process. When I talked to the first buyer, concerned that they weren't getting their refund and asked if they had agreed to the cancellation, they wrote back and said they couldn't find where. Actually wrote back twice because they had carefully looked again - obviously there was no way for them to agree to it.
When I was on the verge of calling eBay it cleared. I talked to some other sellers and one said it had taken 12 hours for their buyer-initiated cancellation to clear. FWIW, when I had a buyer-requested cancellation the previous month (sounds like I have a lot, but it's mostly computer equipment) it went right through, so this is a fairly new problem in my experience.
Not sure if that helps.
“The illegal we do immediately, the unconstitutional takes a little longer.” - Henry Kissinger
"Do not obey in advance." Timothy Snyder "On Tyranny"