Hmm. I don't get some of the comments people are making, but I'll post my guess.
Each of those cards can give enough coins to buy any card in the game? Secret Chamber and Vault discard for coins (so, arbitrarily high, if you increase your hand size), Salvager because you can trash a card to gain the same one (duh), trade route because of the variable value, pirate ship can go arbitrarily high.
I *think* the only remaining card with that property is Philosophers Stone.
I guess honorable mentions to all those things which can *gain* any card in the game - remodel/expand/horn of plenty/masquerade, maybe others. Honorable mentions to things with variable value that can't go quite as high.
But you said said that the Stone isn't the answer. And now that I think about it, Diadem fits that too...
Well, that's my guess. I'll keep thinking.