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The first and last rule of prices is that nobody knows what anything is really worth. Shoppers are guided by shallow clues ("this is cheaper than that") and latent emotions ("it just feels like a good deal") rather than knowledge and deliberate thinking. Smart shopping might be an oxymoron. But smarter shopping? That's a noble goal. Here are 11 tips from microeconomics, behavioral economics, and social psychology to guide you to successful and as-smart-as-possible Black Friday.