Hiya, sorry to hear about your mate's health - hope it only improves from here.
70/80's artists go for somewhere between 500-1900 USD on ebay. The AR500's seem to be valued, not sure what model you have there, look highend and in very good nick? As a guess, if it's a AR500 - locally, I'd say start at around R12-15k and be negotiable.
The Gear Junkie had a similar one w/case at R15k (inc 20% comm), not sure if it sold yet though.
You could put it on consignment at Gear Junkie/Ronnie Allbang & Strumit, advertise it via facebook on SA Music Store (SAMIS) and Revolution Music Trade groups or here on GuitarTalk. And, of course, Gumtree is always an option.
Avoid OLX/Junkmail, scammers paradise. Though that can also happen on gumtree & facebook groups too.
FYI : The Terada factory is the only one outside of the US that Martin licenced to produce Martin acoustics! Though the fujigen factory is better known (I think because they made a lot of fender's for the US market).