A person, during office hours.
WhatsApp is the fastest, and it is answered by somebody who can actually look at a photo of your phone and tell you what it is worth. Mon-Fri, 9am-5pm.
There is no shop to walk into.
The warehouse is in Johannesburg and it is not open to the public. That is a deliberate choice rather than a gap: a retail counter costs money that currently pays for the free courier, and it would only ever serve the few people who could drive to it.
So everything happens by courier, from anywhere in the country, and it costs you nothing in either direction. Collection is free, and if the offer changes and you would rather have your device back, the return is free too.
SellYourGalaxy.co.za is Epic Deals, Tech Revival (Pty) Ltd, trading since 2014.
Or just get the number first.
Most questions turn out to be answered by seeing the offer, and it takes about a minute.