Lifthrasil: When a store advertises with low prices, they have to honor those prices. Otherwise any store would write ridicolously low prices in their catalogues and ads just to get people into the store (where they then can tell them: sorry, the price you came for is not available. But you can buy this or that instead).
Here in Germany it is actually law and there were some successful lawsuits against stores trying to back out of their advertised prices. If there is no 'limited time' clause in the ad, the store has to stick to the advertised price for at least 14 days after the publication of the ad. (Media Markt made a sizeable loss in such a case a few years back. They advertised super-low prices for some pre-order and then tried to declare it as an error. But they were sued successfully and had to deliver).
Stores can set a limited time (like "80% off only on Saturday") or a limited number of sales ("the first 10.000 customers get a whatever free") but they have to publish these limitations clearly visible on the ad. I.e. not in small print. (But in practice lawyers can argue years about what is small print and what isnt).
But if Best Buy didn't publish any limiters with their advertisement, they would be bound to the advertised price in Germany. I don't know about the law in USA, but I assume if some of the customers band together and sue BestBuy, they have good chances of winning. (suing corporations is after all one of the national sports in the USA, if I am informed correctly).
One of the things I love about Germany. They have laws for the people. Not just laws that protect douche bag corporations.