The biggest reason I see the need for an auction house is the problem with commerce.

It's hard to tell what people really need. Therefore even though some items are needed, they are hard to acquire because no one sells them.

This causes strain on crafters, for often times you need to level many crafts in order to level the one you wish successfully.

People think that the sales history associated with an auction house will fix everything, this is probably because it will give players a better way of scoping what's desired.
A sales history has a negative impact on the economy, it sets to define the price of items. Having the prices of every item defined by an auction house makes it very difficult for crafters to make money. If FFXI is any example of this you basically needed to level your craft close to 100 to make any real profit.

Rather then ruining the economy with an Auction House can I suggest that you start by adding a better retainer search function, see how that goes first. This will give us players a better way to tell what items are desired, I believe this is what is missing from the current system that would fill the void that everyone is missing from not having an auction house.

Having a better retainer search will make everyones lives much easier as if you wanted an item you didn't possess and could not find in any wards, you could simply have your retainer search for the item for you. I'm sure someone will fill that order promptly.

Well anyway there's my 2 cents. Please don't ruin this game with a "Sales History", there has to be another way.