Seems to me it's a seller's responsibility to know what he's got. If he's priced it, I have no problem buying it at that price... It's not nearly the same as ripping off grandma for dead grandpa's Honus Wagner card.
My best find of this type was my first two Star Player Candy cards from a longtime dealer's $5 box. He mostly dealt in postwar Topps/Bowman and couldn't be bothered to look these up. I was happy to take them off his hands
(PS: They were raw when I bought them. I later made the genius move to have GAI grade them, LOL.)