Hi Ralf -
Offering a refund is probable the best way to go, but I agree with Frank and the others - if you advertise something as mint, it doesn't necessarily imply it's still in shrinkwrap. I think you're in the right reagrding the auction description, but I also think you're doing the right thing by refunding the cash.
I have an item up for auction at the moment, a Garden of the Plantmaster. It's still in the original shrink, but I wouldn't class it as mint - the
SW has either shrunk or was too tight to begin with so the module is slightly wavy, and the edges have had a few dings. Certainly excellent condition, but a long way short of mint.
Regards
Mike