I try to transfer it to alteil from experience in other games:
I put up a 3* Ruuca in the auction house. I set a minimum sale amount (to exclude abuse there will be a minimum threshhold i can set up for a card of certain rarity) I would like to have this card sold.
From now on for a limited amount of time (eg 3 days) people can give bids on this card to buy it. When the time limit is gone the card will be sold to the highest bidder. If noone gave a bid you get the card back.
In some games there is an additional auction fee or/and a personalising feature to conter unreasonable offers and limit the transfer into a market game

.
Also ther is a need for some kind of bid call back limit, to conter people tricking by protecting a low bid with an unreasonable high one (and calling back in the very last moment).
Auction houses could work /fail dependent on the amount of people joining them. If there are too few offers, chances are high people can sell their stuff for unreasonable high prices and tricking unexperienced ones. Also if you cant sell your cards you start to ignore it and then the system is a fail.
On the other side it could widen the options of free players/low spenders and easen the aquiring of low rarity cards + give the owners an immediate feedback how much value people see in certain cards.
I can't really decide if this something good or bad for alteil :. It certanily would not work with our current not strict policy of alts. Ohh and it could absolutely kill new player expereince.