Customs duty is most commonly applicable to goods being brought into a country. For non-US citizens there are quite strict limits ($100) on the value of goods that can be brought into the USA before duty is payable. (Note that this only applies to goods that will stay in the USA; it does not apply to any item which you'll be taking out of the country when you go home.)
I can find no indication that the US has any scheme for charging export duty on simple personal possessions, and I have never seen customs control on leaving the US. There is an obligation to declare large sums of money and also a procedure for exporting motor vehicles.
You should be more concerned about duty payable on goods being taken into your destination country. For South Africa the duty free allowance is R5,000 (equivalent to a few hundred USD). Hence you need to think carefully before buying high-value items such as laptop computers while in the US.