Very nice. I'd keep it as at some point you'll want one and you'd probably have to pay a lot more for one that sounded as good. It looks like it's got a solid top at least. Can you check that by looking at the soundhole and seeing if the grain runs from front to back?

Does it have a truss rod? It will be under the front of the soundhole if it has one. Have you measured the scale length?

I'd say anything up to AU$600 for a very good condition one. The only one I could find that sold on eBay in the UK went for £57, about AU$100. Others, just called S-50, went for similar amounts, through many had been re-strung with nylon strings.

1967, when the S-50A was introduced, was the year when Yamaha first started making classical guitars, having trained up a team of craftsmen under a Spanish master luthier. These were high-end guitars, all handmade, and were (and still are) correspondingly expensive.

The high price given in that Worthpoint listing appears to refer to a classical guitar with nylon strings, not steel, so probably refers to a Yamaha classical guitar, though if said guitar had a 'laminated spruce top' as stated, it certainly would be a bottom of range guitar and wouldn't be worth much. So I'd ignore it.

I once bought a book on the history of Yamaha guitars, but it was basically just an advert for Yamaha guitars with very little info on the old ones, so I threw it away in annoyance.

Their first steel-string acoustic, the FG-180, came out the year before the S-50A, so acoustic guitars in general were new to Yamaha at the time.

I can find very little info on the S-50A, but it is obviously influenced by classical design in body shape, size and construction. It might be worth putting a phone inside and taking a pic of the bracing pattern used, and see whether that is a classical or acoustic style. There are obvious steel-string modifications, like the steel string posts and a slightly narrower neck. The bridge obviously has a screw or bolt through it to help keep it attached. But otherwise it's certainly a classical bridge, with a straight saddle, so it won't intonate very well. The bridge doesn't have the inlay around the tie-over part that their classical guitars had, so again looks to be more suited to ball ends than classical strings, where lack of binding can cause the strings to wear notches in the bridge.

If you were a lot more local, I'd take it off your hands, but distance and CITES really prohibits it.