What else did the seller have? If the vintage were the minority he may not know.
Just because someone sets up, doesn't mean they are knowledgeable. I was at a show a couple of months back. A gentleman had one t206 among his 50's and 60's cards. I asked the price, when he flipped I saw the back was a Piedmont but didn't fully catch which series. So I asked "and it was a Piedmont 150?" He turned it back over and said "Yes this is number 150 in the set." He apparently thought the number on the back of the card was the same as the number on the back of his Topps cards.
It would have been easy for me to believe that a guy like that could be selling fakes and have no idea.