EVE Online has used the Buy/Sell order method for a long time. I am sure there are better descriptions out there, but I will try to explain it, and if anyone can make it more clear, please do.

To start with.. let assume Adam, Betty and Charlie need Mithril Ore. Currently there is none being sold in the AH. Adam is willing to pay 1g/piece up to 10 pieces, Betty 75s ea for 20 pieces and Charlie 1g50s each for up to 20 pieces. David goes out and farms 40 pieces and sells it, but he wants no less than 90s each. Since Charlie is willing to pay the most, he gets his 20 for 1g50. This leaves 20 Mithril to sell. Adam gets his 10 for 1g each. This leaves 10 Mithril left. Since Betty's asking price is too low, the remainder of David's ore (10 units) gets put on the AH for 90s(his asking price).

Now.. we have a Sell order for David of 10 pieces of Mithril for 90s and Betty's Buy order of 20 pieces at 75s. Edward comes in and will undercut David by selling 50 pieces of Mithril for 50s each. So, since this is under Betty's amount, she gets her 20 pieces at 75s each and the remainder is up for sale. So now we have David selling 10 Mithril for 75s each and Edward selling 30 Mithril for 50s each.

Now, Adam needs 50 more mithril and is still willing to pay up to 1g each. So, he snags the 30 Mithril from Edward at 50s first, then the 10 Mithril from David at 75s each, then this leaves a Buy Order of 10 pieces of Mithril at 1g each.

This cycle continues over and over. The best part is you can see if people actually want an item before you go out to farm it.

Or to be more succinct, check out a financial stock's bid/ask book. For example, go look at Blizzard's order book on Yahoo Finance here: http://finance.yahoo.com/q/ecn?s=ATVI+Order+Book

As one who has made his living trading the markets for a firm for the last decade, I can tell you that I can't even imagine how dysfunctional markets would be with only the "ask" side of the market, and no "bid" side to deal with.

Yes to buy orders!!!!

Some other side benefits are that:

it gives people better insight into what to gather - fathom eel, whiptail or fel iron. Just because the only sell order for an item is high means it will sell for that. But if you get the mat in time you can fill the buy order.

It reduces some of the power of AH bots & campers. Is it really good game play to reward the person/bot who checks the AH every 5 minutes.