For just R150, you can buy a Covid rapid test from an address in Fish Hoek, Cape Town. In nearby Muizenberg, there’s a couple who sell the tests to any buyer who wants to ensure they’re not infectious before they venture out.

Word-of-mouth and Facebook groups help people track down tests, with some sellers offering home delivery. A pharmacy in Cape Town’s southern suburbs was selling the rapid tests for R250. A few days later, they were sold out...