Guichet: what are the rules for Black Friday?

Do you give in easily to impulse buys and promotions? In this Black Friday season, we open a window on marketing strategies and our buying behaviors. What rules must retailers respect when it comes to advertising, displaying prices, discounts or guarantees? What questions should you ask before you buy? And how to keep a cool head when faced with a crossed-out price, and check whether it's a good deal?