Items purchased overseas and shipped to the USA are liable for duty. How much the duty is depends upon the item, new or used, and the value, as well as where it comes from - some countries have favorable rates compared to others. Not every item gets 'caught' by Customs and duty levied, which leads some to believe that there is no such thing as import Duties into the USA.
http://www.cbp.gov/xp/cgov/trade/trade_programs/duty_rates/
There is a limit below which Duty is not charged. I could be wrong, but I recall it being $200 for one item or $100 per item for a maximum of 2 items per day.
There is a table listed in the link above where you can look up the item you are buying and see what the rate is. It is your responsibility to pay for the Duty if it is levied. If the item arrives at your house and is not held up in Customs, then you have no Duty.
This is why some people selling via eBay offer to write 'gift' or otherwise misrepresent the item they are selling when they ship it - they have to fill out the Customs form when they mail it to you. This is illegal, and I would not want to be caught doing it, but people do it.