Sorry to say it, but I think there's a fundamental flaw with this mechanic: if the card isn't very powerful, then there's no real reason to pay attention to it; if it's powerful enough to make the auction "interesting", then suddenly it becomes an overwhelming advantage to open 5/2, i.e. the opening becomes far too luck-based. I can't take the card away from you until I hit $6, and I might not have a better opening than Silver/Silver for attempting to do so.
Also, how do you decide how many Auction cards to put out, and which?
CHAMBERLAIN, Auction-Victory (v2)
While you own this, at the start of your turn, +$1.
~~~~~~~~~~~~~~~~~~~~
Worth Victory Points equal to the number of players.
(the original +$2 effect was too good to pass throughout the game. whoever had it, had a massive advantage, so we were all fighting to get it. even more so during endgame when it became 4 points plus the money we needed for Provinces. the new version doesn't look all that, but if you manage to get, you'll not want to part with it.)
The original version of this was obviously incredibly strong; in a 2-player game, it'd already be like a Harem that you get to play every turn.
HANDMAIDEN, Auction (v2)
While you own this, at the start of your turn, choose one: +1 Card; or trash a card from your hand.
(the original draw until you have 6 cards made Attack cards that make you discard, largely redundant or even help your hand. the new effect should be more versatile without being too beneficial.)
The original version was ludicrous (consider the interaction with Outpost, for example). With this change, it's basically better than Alchemist (since you don't need to find a Potion to topdeck it, nor buy the Potion, nor let the Potion be a dead card) even without the trashing option.
SELLSWORD, Auction (v2)
When you buy Sellsword, place its token on a Kingdom card pile.
While you own this: if you buy a card from a pile with Sellsword, gain 1 Victory Token; if another player buys a card from a pile with Sellsword, he draws only 3 cards for his next hand.
~~~~~~~~~~~~~~~~~~~~
When a card is bought from a pile with Sellsword, you place its token on a different Kingdom card pile immediately.
(altered the Curse gain to draw only 3 cards, which makes for a more blocking effect. other players could still get around this by 'gaining' but not 'buying' from that card pile. the VT gain is also toned down but can still get really powerful if you are able to build an engine that let's you buy more than one card on your turn, and there are cheap Kingdom cards available. however, too many of these can hurt your later hands.)
You probably want that to say "he draws only 3 cards during his cleanup phase". I don't really know how to rate this; it seems like you ought to be able to switch it between two cheap piles frequently and rack up bonus points on megaturns... but then you were probably going to win anyway.
AUCTION HOUSE, Auction
When you buy Auction House, place its token on a Supply pile.
While you own this, cards from a pile with Auction House cost $1 less to you (but not less than $0), and $1 more to other players.
~~~~~~~~~~~~~~~~~~~~
When a card is gained from a pile with Auction House, you place its token on a different Supply pile immediately.
(much like Sellsword but the effect applies to ALL cards, not just Kingdom cards, and repositions the token on 'gain'. not playtested yet.)
I think this cost-altering mechanic is more interesting, honestly.
CITY GUARD, Auction-Reaction
While you own this, if you do not have 4 cards at the start of your turn, draw or discard until you have a hand of 4 cards. At any time on your turn, choose one: +1 Card; or +1 Action.
~~~~~~~~~~~~~~~~~~~~
When another player plays an Attack card, you may discard a card. If you do, you are unaffected by that Attack.
(not sure if this needs to be listed as Reaction since it does not need to be revealed or played or trashed. not playtested yet.)
You really need that to say "at any time during your turn, once per turn," or something to that effect.
Also it should presumably say "if you do not have exactly 4 cards at the start of your turn". Halving the damage from a Torturer is interesting (actually it is not that good against chains in general), anyway.