Just had an idea - a variant of some of the other auction cards that have been suggested here. The mechanic is straightforward, but requires a wall of text to explain.
Thoughts welcome.
Fire Sale
$4 - Action
Gain two coin tokens and put one card from your hand on top of your deck. Before the start of the next player's turn, hold an auction.
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Auction: Reveal the top card of the Black Market deck. Starting with you, each player may bid for the revealed card. Continue left around the table until one player wins the auction. The winner then discards treasure cards and coin tokens to a value equal to or greater than their bid. The winner then gains the revealed card and draws to five cards in hand. If the winner is unable to discard to the required cost, he instead gains a curse.