The evil geniuses at Valve have figured out another way to save you money on Steam.... while turning on a switch that basically just pulls money straight from your pocket and puts it right in theirs. "Buy Orders," are a standing order to buy "commodity items" at or below a particular price.... yeah.
The concept of buy orders comes from stock exchanges (I think they just watched the Wolf of Wall Street) and it's a relatively basic idea. The only really issue in this case is that buy orders can only be placed on commodity items "that are all identical to each other," with no variations in description or function.
Let's break it down. 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.
At the moment only Half-Life 2 trading cards have been set up with this new system, although that list will grow over time, no doubt.
If you want a Metro Cop card, for example, 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. You just know some idiot is going to put down €100 as a joke and accidentally end up with 2,500 Metro Cop cards. 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. Don't be that idiot. You'll just end up with a crazy amount of the same card for no reason.
So, that leaves us with the question whether this is the best thing ever or the worst thing ever. The forums have already ignited. Feel free to leave your 2 cents down below in our comments, but Valve might take it from us for 10 Portal 2 cards in the future.