This is a fair ask, leaning slightly ambitious if the condition isn’t documented well. A new C4 Platinum sat around A$27,000–A$30,000 equivalent, and solid used examples have typically traded in the A$13,000–A$16,000 band, so A$13,500 is right on the lower end of normal for a clean pair. If these are genuinely excellent cosmetically and electrically, the pricing is sensible rather than flashy.
The appeal here is the C4 Platinum’s scale, speed, and Dynaudio’s usual tonal control: big but coherent bass, very refined treble, and superb imaging when properly driven. They’re high-end, heavy, and not common on the used market, so value is partly in getting a world-class speaker without paying new money. At this level, I’d just want confirmation on driver health, crossover history, and that the original grills/outriggers/spikes are included.
Independent perspective — not a price guarantee. Always verify condition, accessories and provenance before purchase.