September 22nd, 2002, 6:47 pm
The average price doesn't enter into it.All orders are limits, there are no stops. So you either input the maximum price you will pay to buy, or the minimum price you will accept to write. Of course, you can put in multiple bids with different quantities at different limits.The computer scans all the orders and picks a price P such that the total amount of buy orders with limits >= P equals the total amount of sell orders with limit prices <=P. Those buy and sell orders get executed at P. All other buy and sell orders get thrown away. I don't know how the system deals with a range of possible market clearing prices or the fact that the totals will not match exactly at any price. These details are not hard to manage.Manipulation opportunities are minor. You can certainly submit bids or offers that you think will lead other people into moving the market in your direction, then cancel them. For example, you could put in a big bid to buy a $100 when the market price is $90, hoping that will lead people to bid $95. Then you could cancel your bid and put in a offer to sell at $95. But that counts on other people being fooled. I imagine the auction monitors for this kind of thing and kick you out if you do it reguarly.If there aren't a lot of bids and offers, you can pick your level cleverly. Say you want to sell one unit and you see only four items: bids at $100 and $102 and offers at $98 and $100. If you don't bid, all four will be filled at $100. If you offer at more than $100, it will be ignored. If you offer at less than $100, you and the $98 seller will be filled at whatever price you bid (unless you bid less than $98, in which case everyone will be filled at $98). So you would bid $99.99. But this assumes no one else changes. The $100 seller might react by offering $99.98. Or someone else might come in at $99.