We need auction house, to thwart scammers and market manipulators.

At this point, from what I have seen:

20% of people trading are just trying to scam you (give wrong orb, try to put in wrong items, etc.)
60%+ of items on the market are not for sale. Whether they are to manipulate the economy or just too busy to ever sell, well more than half of requests go unanswered.

This leaves about 20% of trade working for its intended purpose. An action house removes each of the above, which are major reasons why trade is terrible and new players end up leaving.