I noticed some questions going around as to why there is such a difference between the buyout price and "last 10 sales" price when searching commodity auctions, and in particular why your buyout price doesn't change.

Your buyout price doesn't change because it is too much work for the server to calculate a precise buyout for your number of items. The buyout doesn't update on each search because if it did then people would keep researching to try and get a better deal, and having so many people do this melts the server.

Instead, the game gives you a static buyout price that lasts for a certain amount of time. This price is higher than the "last 10 trades" price. When you buy your items you will get two returns from the auction house. One will be your purchase, and the other will be the refund of however much you paid in excess of the going rate for the items you purchased.

I tested this today. Result from purchasing a single square amethyst: http://imgur.com/r5E6h

This also means that if you want to aim for a quick sale of your commodity items, you want to go for something between last 10 trades and last day's price (once that AH has been up for a day...)

Hope this helps clear things up for people.