As a seller myself, I would simply state that you have no problem providing a refund once they return the item. It's as simple as that. Like I said, they know that if they go through ebay or paypal for the refund they will be expected to pay for tracked shipping which is surely far more expensive and not worth it.
In this day and age there is no reason why someone can't provide a picture (cell, webcam, camera etc), it sounds like they are flaking out.
Hopefully someone can give you an idea of how much post is.