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Buying and selling the same stock
Elaborating on kelsham's answer: You buy 100 shares XYZ at $1, for a total cost of $100 plus commissions. You sell 100 shares XYZ at $2, for a total income of $200 minus commissions. Exclusive of commissions, your capital gain is $100 for this trade, and you will pay taxes on that.  Even if you proceed to buy 200 shares XYZ at $1, reinvesting all your income from the sale, you still owe taxes on that $100 gain.  The IRS has met this trick before.