I wouldn't advertise a good running 16 inch Chonda for small areas like a Unit because of all the ones I've had will last years and cut half an acre of grass for years , that's all the Chonda mowers that get oil changes + air filter cleaned and are good mechanically when I get them.
A lot of people don't realize when they message you on gumtree that you can click on their user name in the message and it goes straight to their Gumtree account so as long as you keep their message you can wait until they are listing items for sale and then message them that you will buy something they are selling at half price but some you can message straight away as they have for sale ads at the time.
I was selling a good Vintage mower for $100. and had a guy message me to buy for around $50 and when I looked at what he was selling he had the same mower that wasn't as good and was asking $250.
If you go into your email account, not the gumtree messaging account you can block them from ever sending messages again ,that's the easiest thing to do with lowballers.
As we know Phone messages are easily blocked too.
It's kind of funny what people do as I've had buyers fill the tank with diesel and one guy took his mower to a service station to ask what he should put in the fuel tank and the guy at the shop sold him a bottle of engine oil so I get a call that the mower won't run.
Sometimes I give buyers a free 5 litre fuel can just because they have no idea where to buy one from.