The great minds at Valve have figured out another way to save you money on Steam while sucking more of it out of your pocket than ever before: "Buy Orders," a standing order to buy "commodity items" at or below a particular price.
The concept of buy orders comes from stock exchanges and it's a fairly simple idea, really, the only complication in this case being that buy orders can only be placed on commodity items, those "that are all identical to each other," with no variations in description or function. You place a standing order for a certain number of items at a certain price; if it goes on sale for less than that, you'll get the lower price, although those offering higher prices will be given priority on newly-listed items. In cases where there are multiple orders offering the same price, they'll be filled in order from oldest to newest.
For now, only Half-Life 2 trading cards have been designated as such, although that list will grow over time. So if you want one of these Metro Cop cards, for instance, you simply go to its page, click the "Buy" button and then specify how many you want and how much you're willing to pay for them. Multiple buy orders, to a maximum of 10 times your Steam wallet balance, can be placed and they can be canceled at any time, but they'll also remain standing until they're filled. All sales are final, so try to keep track of what you're doing.
The predictable debate about whether this is the best thing ever or the worst thing ever is already well underway in the Steam forums. I can't even begin to predict how this might impact the market, but if this sounds like action you want a piece of, the fray awaits on Steam.