I use Gumtree all the time, have sold about 30 items on it, including a $30,000 car.
My add always says
"CALLS ONLY. Txt and emails ignored"
Funny thing is, women will always send a text, even though they read the add, then call the next day saying that they had sent the txt, to which I say, yes and I ignored it.
Blokes, well, they always call.
The dodgy folk will always ask a question I find, so I put all the info you would wish to know about the item for sale plus heaps of pics, then there is no need to ask questions.
Oh, and at the same time, there are 2 types of calls.
Item for sale is $25, a kids slide.
Call 1. Hey coussy bro, you take $12 for it??? (think kiwi accent)
Call 2. G'day mate, whats ya address, and when ya home?