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How often do you see someone using a stand-alone pocket-sized calculator these days? Unless you work with engineers, economists or mathematicians, spotting one is akin to seeing a unicorn. But, as it turns out, there is a market for calculators, especially graphing calculators (which are required, for instance, for some math classes in high school and college). Before you toss your old calculator, check CalculatorSource to determine its value and popularity. Why that site? Reseller Ratings gives them an 8.4 out of 10 in customer satisfaction.