If you have a tracking number for the parcel that clearly shows that the item has been successfully delivered and signed for then contact the buyer and ask her to open an Item Not Received case in the eBay Resolution Centre, if she hasn't already done so. Once an Item Not Received case has been opened leave the case open until the buyer escalates the matter to eBay, in which case once they see that the tracking proves successful delivery of the item to the address you were told to send it to the case will be decided in your favour and the buyer will be unable to leave feedback. If she leaves negative feedback prior to you winning the case then once the case has been closed in your favour contact eBay Customer Services using the Call Me Back option and request that the negative feedback be removed, as eBay decided the Item Not Recieved case in your favour and therefore the buyer shouldn't have been able to leave feedback in relation to the transaction. eBay will remove it for you if you do indeed need to do that.
The other option you have available to you is that once the buyer has opened an Item Not Received case against you, whether you encouraged her to do so or she did it without any persuasion from you, contact eBay Customer Services and inform them that the tracking proves successful delivery of the item and ask the eBay representative you are speaking with to put a trace on the tracking number and have a look at the details regarding delivery of the item. Once the eBay representative sees that the tracking proves successful delivery of the item he/she will close the case in your favour, the end result being that the buyer won't be able to pursue the matter via eBay.
In the event that the buyer subsequently opens up an Item Not Received case via PayPal they'll decide the matter in your favour once they check out the tracking details and see that the item has been successfully delivered. You've really got nothing to worry about as you can't lose an Item Not Received case when the tracking proves successful delivery of the item to the buyer.
In addition to the above infomration, make sure that you keep that receipt with the tracking number on it for at least twelve months after the date that you sold the item to the buyer just in case the buyer tries to do a chargeback, alleging unauthorised card use. If she does try to pull that stunt then give PayPal the tracking number again, but make sure that you also report the buyer to ActionFraud and then provide PayPal with the crime reference number. The buyer would be guilty of trying to defraud you of your money if she were stupid enough to try and pull this stunt, so if she does try it and you follow this advice then she'll find herself in very hot water when ActionFraud decide to investigate the matter furhter to find out exactly what is going on if they suspect that she's trying to obtain money via fraudulent means!
Finally, if you have not already done so, add the buyer's User ID to your Blocked Bidders List. If you're unfamiliar with how to do so click on this link https://offer.ebay.co.uk/ws/ebay/isapi.dll?bidderblocklogin and add her User ID to the white box that appears on the screen. Once you've done that click on the Submit button beneath that white box and the buyer will be unable to purchase anything else from you in the future.
