Hmm.
This has happened to me before. Royal Mail will swear black's white that they left a card and therefore the non-delivery was the recipient's (i.e the buyer's) fault. The buyer says otherwise - that the postman forgot/was too idle to leave a card and therefore she had no idea that the delivery was forthcoming. You have no real way of proving it either way.
Did your parcel have your address on it (as the sender). I am now very careful to always write my house number and postcode on any parcel I sent. If so, you should get it back in due course. If there was no address on the parcel, then you may well be out of luck.
It all depends on how much your buyer wants the delivery. If she is prepared to wait until RM deliver the parcel back to you, you can always repost it (probably at your own cost). This is what I did when the same happened to me as my buyer really wanted the items. In the meantime, you need to contact Royal Mail's customer service yourself and ask them for assistance in tracking your parcel. I haven't found them particularly helpful in the past though, so be warned.
Or you could just refund now and put in a claim for compensation. If Royal Mail are being unhelpful in trying to find your parcel, this may galvanise them into taking action. I would get an email from your buyer confirming that she definitely didn't get a card through her letterbox and include a copy of that with your claim form as that places the blame on Royal Mail so they then can't turn around and say, oh well, we tried to deliver, it's not our fault....
FWIW, I post most things by Recorded delivery these days and always send my buyers an email telling them what the tracking number is and advising them to get in touch with their local sorting offices should the item not arrive within a few days. And always put your own address on the outside of the parcel.