Would you buy a knife from the factory in that condition as "new"? I know I wouldn't. I assume that this came from ebay. Unless a pic of this blemish was included in the description, then it appears that the seller left this out on purpose.
For the seller to list it as mint is a tragedy. Maybe it came from the factory that way, but at best I would label it a "second". If the seller has any morals at all, he will refund or discount the cost.
As far as what constitutes "mint", that is a discussion for the ages. Now mint, to me anyways, would mean "if I were to buy the knife brand new in a store and had a chance to look it over and inspect it, what would the quality of it be"? I assume (rhetorically) that most to all knives come new from the factory with no nicks, scratches, or visible un-natural blemishes.
A decent seller will clearly list all visible issues with a knife, and clearly show pics of those issues. Most good dealers want happy customers, and can't have a good business or reputation by having deals caught up in the red tape of disputes or refunds. So by them clearly stating the condition, they can avoid 99% of the grief.
That said, I have seen very few absolute "mint" knives, there is always something you can find on them if you look hard enough.
Buyer beware, and don't forget to ask for additional pics of the "unshown" parts of the knife. Beware of a seller that will only show one side of the knife in their pics.
Also, I am a firm believer in promoting the good dealers and exposing the bad ones. I'd hate to see someone get shafted similar to what happened here. let us know how it turns out.
Glenn