I purchased a set of expensive Swarovski flutes glasses on Gumtree and they are not as advertised (not even crystal knock-offs). The seller has blocked me and I have only a name and bank account.
The other option is to contact Gumtree for contact details of the seller and take them to small claims court over the matter. As far as Australian Consumer Law specific things you can do it was a private purchase so you don't have the protections available that you would if you bought it from a shop.