Have the buyer e-mail you a digital photograph of the damage. That's the only way you (and your conscience) can verify what is wrong with it and whether or not you may have overlooked that prior to shipping. Even the best packing won't keep some gorilla from hurtling your package in ways you never thought imaginable...that's why on bigger ticket items I always cough up and pay the insurance (just in case).
On the buyer's side, some people are really, really picky and will call out every minor defect. If the price of the item was reasonable to begin with, and minor imperfections are part of the component, then some level of balance must be achieved to start. If you don't believe the buyer is being truthful or reasonable regarding the purchase, then you need to confront them accordingly and tell them like it is.
Worst case is that the buyer hunts you down and kicks the crap out of you.
Or if you feel like you were not entirely upfront with regard to condition, you offer him a spot discount on the purchase price, send him a check, and be on your way. $#it happens.
Good luck,
-Dean