OK, in the first instance this is my fault: I bid on a faulty iMac G4 a week ago, intending to use the motherboard to repair another, which I've been trying to sell in faulty condition for several weeks. On Thursday afternoon someone paid the asking price for the one I already owned, which meant that I didn't need the other one and didn't want to spend £20 and several hours travelling to pick it up if I won it. So I immediately contacted the seller and requested bid cancellation, with apologies. In the past people have done this to me several times, and I've always agreed because there is really sod all you can do to enforce a sale on eBay. No reply, and nothing happened.
Tonight the auction ended, with the computer bid up to £19 and me still the winning bidder. I've contacted the seller to point out that I'd requested cancellation, and again asking them to cancel the sale; they've sent me an email saying I've won it, they aren't prepared to cancel.
So I've pointed out that they can either cancel the sale now and offer it to the second bidder for whatever their final bid was, probably £18, or wait a few days and open an unpaid item case and eventually cancel the sale. That'll leave me with an unpaid item strike and probably negative feedback, but to be honest I don't really care that much. It'll be interesting to see what happens.