"Mint" is widely used in the philatelic world to describe postage stamps in unused condition (ie with the original gum, never been stuck to an envelope, etc.) The term is also commonly used in the sports card and comic book collecting worlds.
So it isn't surprising that the term occasionally leaks over to the paper money collecting world. It's natural for stamp and sports card collectors to assume that "mint" would be used for paper money too. Granted, it's not a proper term, but let's give people the benefit of the doubt and first assume that they are simply inexperienced, rather than trying to pull off a scam. Buyer beware, of course.