It appears that selling copyrights of articles produced by you is considered ordinary income to you. You might review this blog article by a law firm that specializes in this type of law.
The tax rate will depend on how much income you end up with (6 figures covers a lot of ground) and your filing status. You will have to add your income from the sale to your other income and subtract your Standard Deduction (if that's what you use), to get at your taxable income.
Take a look at TurboTax's Tax Bracket Calculator to get an idea of what your tax rate will be.
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