Here's my guess:
They are all cards whose functionality is changed or "broken" in some sense if they are in the Black Market deck. If you get Cultist or Knights from the Black Market, you have the only copy in the game, so the card will never play/trash a copy of itself. Transmute and Rats can never gain copies of themselves if there's only one, and Treasure Map can never activate if you only ever have one. Fool's Gold doesn't have its functionality changed completely because you can still Counterfeit it, but there's no other way to play two copies (and this trashes it). Peddler will always cost 8 from the BM deck, but that's not related to their being only one copy of it. Duchess is the missing card, because you can never use the "gain it with a Duchy" ability because it's not in the supply. If I'm right, I'd argue that Peddler might actually belong on the list, but it probably has to do with the specific wording of what you had in mind, which I don't think I'm smart enough to figure out.
Edit:
Oh, and Black Market is dishonorable mention because it's the only kingdom card that CAN'T be in the BM deck.