if you mine and sell then it's an income and you would pay income tax - like if you farmed a field of carrots and sold them

if you buy BTC then sell at a profit, then it's a capital gain, like if you bought some stock.

No tax if you buy goods with your BTC, then the vendor might have to add sales tax before pricing in BTC (this is less clear - because it depends whether the vendor is using BTC as purely a medium of transfer and then using e.g. bitpay to pay them in USD)
 
That's how I see it, anyway.

Will