I think the issue is that they are claiming the goods to be faulty, you have inspected them and deem them not to be - so it's a matter of opinion what you can and can't prove.
As they have been sent back as defective it doesn't really matter on the state of the packaging. If however they have been sent back as unwanted, and they are not in a resaleable condition that's a different matter (as long as your site T&C's support this)
We do an inspection report document for so called faulty items that we deem not to be, and even charge for sending the goods back. The report states that we will dispose of the items after 28 days so the ball is put firmly back in their court.
But as others have said the real decision is down to how much grief they will cause vs what you can be bothered to absorb.