I don't like the idea and here are my 2 simple reasons:
1. It makes the game more complex and harder to understand, but not more interesting or more funny. The treasures are treasures for a reason and it is nice that we can play them ONLY in the buy phase, so we must be prepared to do so.
2. It makes combos that seems a bit silly to me. The general idea of Tactician is "I will change 2 good turns for 1 hell of a turn!" With Vault, you are still punished the way you cannot play Golds and Silvers (and you need a Village). With Black Market or Auction, the idea dies.
I don't like Black Market, but I don't hate it for 1 simple reason - IT IS A PROMO! The card is wierd, but it should be wierd, cause that is to me what promos should be.
So if this is an "I think it would be awesome to have a card like this in an expansion" idea, I totally disagree.
If this is an "I think this card is funny and I want to play with it with my friends" idea, it is of course your bussiness
I don't find it funny, because it adds nothing new to the game (except some annoying
AP such as "I have a Library and a Remake in my deck, so should I put the Coppers in game or not?")
PS: Sorry if this was a bit harsh, I didn't mean it