Gumtree is much the same platform in Australia as what Craigslist is in the USA.
What I can say is that it's the way people have become all over the world, but you'll still have nice respectful people as well, sadly as time goes on those respectful people have had a gutful of how life is becoming and are slowly becoming another one of those disrespectful people.
I've found that when I sell something I will advertise it at a sensible good value price and initially all I'll get is the half price flippers that respond but I just ignore them and then a few days later I'll get a sensible person respond that will purchase the item and make the experience all worth while.
I strongly feel that the way of the world with it's high unemployment and many other social issues has made folk become rude and arrogant people, sadly it aint going to change for the better, so get used to it and one has to become far more resilient.