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)
In the US, if you buy goods with your BTC it is possible you could trigger either a capital gains tax or possibly need to report it as a Schedule C (essential it could be viewed as a barter transaction).