Pictures are considered a full description if there is no text stating otherwise. Since the seller stated the knives as mint, he misrepresented them.
If you have two descriptions contradicting each other, then it is automatically not as described. End of story, the seller messed up.
You can blame it on the buyer, but there will always be another buyer that will do the same thing. If the sale ad is wrong from the start, then you can lay no blame on the buyer.
Should he of paid closer attention? Yes, but people take a sale ad at face value all the time, without inspecting little details. If this was blade forums, and someone missed a flaw in the picture, people would not let it slide on a NIB sale.