Gumtree review: Shonky customer service and blatant lies

I purchased a set of Pirelli tyres from Zac Gummie since 2017 on Gumtree Australia. He failed to advise the tyres had been used for drifting which altered the composition of the rubber to the point no tyre fitter would fit them to our vehicle. When I requested a refund from Zac he blatantly refused. Greg at Gumtree alleged he was the police liaison officer and asked me to contact the police to contact him quoting reference Case #00428055. Despite contacting Gumtree dozens of times and complaining on the Gumtree Australia Facebook page Greg seems to have gone walkabout. Unfortunately I cannot progress the complaint because Queensland Police claim it is a civil matter and QCAT won't accept a complaint unless accompanied by Zac's residential address. Why use Gumtree when eBay have safeguards in place to protect consumers and sellers?