The last time it happened to me was quite a while ago. I think my purchaser was genuine but you can't always tell. I refunded her money and made a claim through the Post Office for the lost item.
See
here for additional info: For lost or damaged items that have an intrinsic value, with evidence of posting with Royal Mail (such as a certificate of posting) and evidence of value (such as an invoice or receipt) customers will be entitled to a postage refund plus compensation for actual loss up to the value of the item, or 100 x 1st class stamps at the first weight step, whichever is the lowest.
The PO were pretty good and quite quick but I thought it simpler to refund her first and then sort the claim for myself. I didn't want to risk negative feedback but I'm fairly sure the guidelines on feedback and complaints have changed now. Your buyer will need to open dialogue in the resolution section - Paypal operates a similar process but she'll still need to report the item not received. Did you mark her item as 'SENT' when you posted? It's good to have the additional info.
Whatever happens next I'm sure you can ask her to wait a bit longer. I'm not sure what is happening with Royal Mail at the moment but I'm still getting warnings about delivery delays for industrial action.
For future reference there are some high risk buyers - based on category or feedback. If in doubt always use Recorded Delivery and build it into your costs. If my item sells for much higher than I think it's worth I'll always pay the extra for free, it gives me peace of mind. All electrical items are sent special delivery as these can sometimes be dodgy or unreliable buyers too!!