I asked a former customs inspector who works in my office - ultimately it is up to the discretion of the postal inspector whether to charge duty or not. There is a general schedule of tariffs, but it's impossible to explain in a forum like this - the amount charged depends on the country of origin and the type of item being imported. As a very vague general rule, used items being imported for personal use of relatively modest value will not be subject to duty, but it all depends on the item and the value. A used Rolex is still probably going to be charged import duty.