Of course in our fictionary MMORPG we could use a bog standard auction house system to trade cards. But then a lot of bad things would happen, things you can observe in Magic the Gathering Online, and in World of Warcraft: People would trade not to get the cards they need or get rid off the cards they have in surplus, but they would trade for profit. In MtGO it quickly became impossible to trade one card for another card of the same value. People only traded for "tickets", the MtGO currency, and trading was ruined by people buying everything cheap and selling it on at a profit. And in MMORPGs like World of Warcraft we are all aware of gold farmers, leading to negative secondary effects like botting and keylogging.
All this is caused by greed, and trading with greed in mind is made possible by asymmetric trading: Nothing has a fixed value, and so people can overpay, underpay, or even "trade" you 1,000 gold for nothing in the game (while receiving $50 outside the game in exchange). All the games using standard trading and auction house systems suffer from this, so we need a radical solution to make Shandalar free of RMT and profiteers: We will restrict all trades to only allow symmetrical trades.
This is possible because right now we only have two things in the game: points of various colors, and cards of various color and rarity. And we have NPCs who sell those cards for points, so we KNOW how many points every card is worth. Thus we can restrict all trades to symmetric trades, where both sides put up cards of exactly equal value. We don't want people to be able to trade points, or trade cards for points, because then questing becomes pointless. We just allow trading cards for cards of equal value.
Thus our "auction house" looks very different from a classic MMORPG auction house. You can have both buy and sell orders. In a sell order you put up a card you'd be willing to trade away, and list one card, or several possibilities, of equal value you'd want for your card. A buy order is just the reverse, you say what card you want, and list several possibilities of cards of equal value you'd be willing to trade in. We might want to add an auction house fee in points, but points don't change hands between players.
As you can't trade something for nothing, or even something for something that is worth much more, it will be extremely hard to game the system to trade cards for real money. After all, we want to have the monopoly in selling cards for real money, in the form of booster packs. And because there are no profiteers gaming the system, players can trade their cards in fair trades with other players, without anyone feeling cheated.