Is it as simple as buying from a local site, or you have to factor in duty and that?
It depends. Some items on Amazon are sold by Amazon.com LLC and some are sold by third party sellers - like on Ebay or BidOrBuy or Kalahari.net. Of the items Amazon.com carries they only ship CDs/DVDs/Books/BDs to SA. Clothing, electronics, etc. are not shipped, except for a small subset, which is enrolled in the Amazon Global Program. I think Kindles are enrolled there.
Of the items they ship, you can prepay duties with them or if they ship by post office, you may get lucky and not be charged VAT and duty. When it comes to third party sellers some of them do sell electronics to SA and these items go through mail or courier. You end up paying SAPO the duties or the courier company.
Amazon has recently started to suck. They now obey some licensing restrictions on CDs/DVDs/BDs where the American publisher only has license for American release. Some of these items which are not allowed sale outside of North America, will not get shipped to SA. Other shops are cheaper and ignore this rule. My Bluray collection includes items with "NOT FOR SALE OUTSIDE OF USA" or "LICENSED FOR USE IN US/CANADA ONLY". Amazon shipped these in the past, now for some of them, they don't, but other shops are usually cheaper and don't mind shipping these anyway.
Note you can use the same password/username on Amazon.De, Fr, Ca and UK, as .COM. Amazon.JP requires a separate account.
Amazon is one of the easiest sites to use. You can also buy stuff for third parties. If you have a relative in France, you can go to Amazon.com or Amazon.fr and send them a gift, using your billing info and CC details.