The price history needs to go a lot further than the past 20 auctions in order to prevent price manipulation.
Blue Gartr Interview:
http://www.bluegartr.com/threads/107497-FFXIV-Naoki-Yoshida-Interview-Transcription
While having a price history is great, and a very good first step, you can't stop with only 20.some of the major changes we will be making includes a more advanced player search and to get a market system as close to the Final Fantasy XI auction house as possible. This would be where you could search for an item, and then buy that item, or see the past 20 auctions.
With such a limited history, goods can be repeatedly bought and sold for non-market value prices on purpose by a player or group of players in order to artificially inflate the price of a particular good.
For example:
Say an Iron Axe is normally sold at the wards for about 10,000 gil. Anyone checking the price history could see that the past 20 axes have been sold for around 10,000 gil. However, I come in and repeatedly buy and sell the axe using a friend or an alt for 25,000 gil 20 times. I then list all my Iron Axes for sale at 25,000 and any new buyers and sellers looking at the price history think that the axe is worth 25,000 gil when it really isn't.
This was somewhat of a problem in FF11, and it can potentially be a problem in FF14.
There are a few differences and safeguards, however. I assume that the item search will still be around so that people can see the actual prices that the players are listing their items, so if the price history shows a history of overpriced sales, but someone still lists their item at the normal price, then there's nothing stopping the customer from getting to that purchase.
This is better than the FF11 auction house because the ambiguous "auction" part of the transaction is eliminated, which cuts out another avenue for price manipulation due to customer ignorance.
When I see this quote:
I hope that it just doesn't carry over the features that made price manipulation possible.to get a market system as close to the Final Fantasy XI auction house as possible.
I would keep item search, let people buy items directly from the item search menu, and then implement a price history that goes back for an entire month, at least.