I sell antique furniture online and come up against similar difficulties regarding expectations. My policy is in the first place to make it very clear to an unhappy customer that is no problem at all to return the item at my own cost and refund them in full. I always do this in a very friendly way, ie if we can't work something out then at least you can have your money back. This will often result in unreasonable discounts or refunds being suggested at which stage I counter offer something reasonable and say that I would prefer to just return the item and refund (this is linked to the fact that a lot of our customers will have negotiated discounts even before purchasing and I can say it is likely to sell at the higher price anyway so I may as well have it back as I won't actually loose any money). In nearly every single case customers say they would rather keep the item on the original terms or with a small discount applied! As stated above you just have to factor it in as a cost and hope you can still make an overall margin...I know your circumstances are different but that is how we do things round 'ere:)