This is difficult to answer. This is what i know

Some of these keys are original, purchase in bulk usually with corporate or university discounts, and some entrepreneurial guy who has access to these special discounts figure out that he can make money by reselling them. Technically is legal in many countries, because they purchase from microsoft legally (although with big discount). and some countries' courts told MS this can be resell by law

but you may have problem with support from MS