Speshpaul – failure to declare items to customs, or lying is a criminal offence, which carries a possible jail sentance. Customs have more powers than the police, and don’t like being lied to. My job requires an airport pass and security check. If I was found in flagrant breach of customs regs I would be unable to maintain my airside pass, and lose my job.
If people want to risk it, go ahead. Chances are you will get away with it. But for me, and many others it simply isn’t worth it.
With reference to US state sales tax, buying mail order in the states, you still pay state sales tax in the state where you are posting to, so not an easy steal there either.