When I advertise (which isn't often) I make sure a full description and all info is included, I have researched the price and clearly stated all red lines, ie no pay pal etc.
Then you just have to sit back, deflect the jokers and wait for a real customer to come along. There have always been dickheads, which is why many ad's from the 60's on were inclined to state, "No time wasters".
I'm sure it's worse these days with the internet, but you have to have patience, stamina and not need the money in a hurry, take your time.