The stores often have extra dust bags lying around and if the sales associate is friendly enough they'll probably give you one if you carry your bag in and say you lost the dust bag, or some similar excuse.
Back to this buyer though - if you included the dust bag in your listing description, I'd kick it back to her or escalate to eBay, advising that she didn't return the item in the same condition (condition being: with its dust bag), and have her send it to you at her expense or accept a partial refund deducting whatever reasonable amount. But I'm petty AF, especially if you said the buyer was rude. [emoji854]