In all areas of collecting, from movie memorabilia to oil paintings, something is authentic if its true identity is described accurately and sincerely.
If you pay good money for an “original 1930 Greta Garbo photograph by the famous Hollywood photographer George Hurrell,” you expect to receive an original 1930 Greta Garbo photo by George Hurrell. You don’t expect a 1970 reprint or a photo by an unknown photographer.
An item does not have to be rare or expensive or old to be authentic. It just has to be accurately and sincerely described. A cheapo 2003 reprint can be authentic if described as a cheapo 2003 reprint.
Errors in the description of an item are considered significant when they significantly affect the financial value or reasonable non-financial expectations of the buyer. An example of the reasonable non-financial expectations would involve a collector who specializes in real photo post cards of her home state of Iowa and makes it crystal clear to the seller that she only wants postcards depicting Iowa. Even if there is no financial issue, she would have reason to be disappointed if the purchased postcard turned out to show Oklahoma or Minnesota.
Many errors in description are minor and have little to no material effect. If that 1930 Greta Garbo photo turns out to be from 1934, it may not effect the financial value or desirability to the purchaser.