1. "How much is the sales tax in this state?" or "What is the sales tax in this state?"
2. I don't understand what you mean. Perhaps "Why are you sitting at that computer? You know ...."
3. I'm not sure what you mean. If by "BS" you mean the dung of an uncastrated male bovine, and the import is that you doubt the veracity of a claim, the sentence is natural vulgar colloquial.