Okay, it seems that things have changed.
I have about three cards that I want to cash in (basically they were bought at various intervals when airport collections did not pan out, and two have expired albeit with cash on them. They only last XX years).

I flew out on Monday, and having time on my hands, went to the kiosk at the airport and asked to cash them in - they wanted the receipts!!!!!

So, you cannot sell them back unless you have the original receipts. I will inquire at Sandton station Sunday when I get back - they told me that you complete a form and you are good to go, so I shall see.

I apologize for any confusion.