Sold item on eBay, buyer didn't pay after multiple reminders need some

So I put my old broken iPhone for auction on eBay. A member won it for $8.50 (it was very broken so it wasn't really worth much) plus $5 for shipping.

In my other listings I included that I expected payment within 24 hours after the auction ends, however I forgot to include this in the iPhone's listing, so I gave the buyer 48 hours to pay. I even sent them 3 reminders, and they didn't respond at all. After the 48th hour, I opened an unpaid Item case, and waited another 2 days for her response. Just yesterday, 8/6/2015, did I receive a reply that she would pay.

Today, 8/7/2015, she finally paid, but I was fed up with her not answering my messages so I just refunded her money through PayPal.

I tried canceling the order on eBay, but it keeps telling me, "We weren't able to cancel your order
You can't cancel this order right now because there's a case open for it." The Unpaid Item case was closed automatically after she paid as I used the assistant.

Any one ever had this situation? How did you deal with it? And how can I get the order to be cancelled?

Appeal to the ebay authorities. They'll handle it.

It seems to me you were very hasty in refunded her money. You can't just do that. You have to consider what is reasonable. Has it been a week? No, not yet? Then ride it out (yes, even with your terms of 24 hour payment). Once they pay, it becomes a regular transaction just like any other. It's not her fault you refunded her money. I'd say technically she gets the money and the phone now because she paid and you didn't want it.

Not everyone is technically savvy and not everyone checks e-mail once a day. The fact that no one responded to 3 reminders in 48 hours (which is obnoxious also) is not a reason to cancel a transaction.