I think I've gotten at least one off a dusttrap in the Ancestral Lands, and I imagine the Keycutter Tonic will occasionally produce one, too. So, the seller isn't necessarily someone who picked it up at a party.
What I want to know is what the Yellow-Teal Square Key opens. It pops up on the auction list if you look for "keys" in the search box, but I have never seen one in the game, nor a door that is opened by one, that I can recall.