They probably keep it to use for the next RMA replacement, I have no idea. They’re not obligated to give a new in box replacement.
There’s no VAT or tax or anything as it gets invoiced at R 0.00.
That's strange because when I had replacement RAM sent to me by a US company I had to pay tax even at Rand 0 value. SARS don't accept that. I suppose businesses have a way of going around that.
But for someone doing a refund as in B2C the device would get sent to his address in the box and all and the value may still be marked as what it is with an annotation of replaced merchandise.
I've had a few instances of companies sending replacement parts and items (from the UK and Japan) when it flew under the radar
but I think I was just lucky then.