Unfortuantely as you didn't use trackable postage,Paypal won't cover you.

JMO but a couple of things you CAN do although you won't necessarily get your money back.

Contact her other sellers.

If you've already left positive feedback,leave a follow up ie 'Is now claiming unauthorised use of Paypal account'

Send registered letter to her plus an email re you want immediate payment for the item or you will report to Online Fraud.

Report the buyer to Online Fraud if they refuse to repay you.

Is the dispute a chargeback from her bank or is it alleged hacking of her Paypal account ?

Perhaps you could ring Paypal as well.

Nothing else to suggest sorry.