The "Mandela coin" refers to the circulating R5 coin featuring Nelson Mandela's portrait, issued by the South African Reserve Bank (SARB) since 2008 (and updated in 2018 for the centenary).
The South African Mint issues separate collectible Mandela coins (gold, silver, proof sets) with certificates of authenticity. These can be sold to coin dealers, auction houses, or via the Mint's buy-back program—but they are distinct from the circulating R5.
If you have a standard R5 Mandela coin from your change, spend it—it's worth exactly R5. There is no special market in Pretoria or elsewhere for selling circulation coins.