I have used ebay, gumtree, facebook and freecycle in the past. Can't particularly think of any good stories, but at least a few times I've had people buy things on ebay for £2 collection only and see they live 100 miles away. Send them a message - when are you collecting? Reply "can I have my money back?"
I think the first things is to know why you're doing it... if you are looking to clear space then just get things out of the door. If you are raising money then maybe take a different approach.
I think that the key is to be clear with the advert and then just reply briefly once, or simply ignore, the people who don't appear to be good at reading or comprehension.
If the price is firm then make it clear it's firm before they turn up and literally shut the door on them if they start bartering.
If it's freecycle make it clear that it is first come first served (but obviously if someone says they will collect at 2pm you won;t give it to anyone else before 2.30). Make this explicit - I AM CLEARING SPACE. THE WARDROBE NEEDS GOING ASAP. YOU'VE SAID YOU WILL BE HERE AT 2PM, IF YOU'RE NOT HERE ON TIME I'LL BE LINING THE NEXT PERSON UP TO COLLECT AT 2.30. PROBLEM? WELL TURN UP ON TIME OR PASS ON THE ITEM AND I'LL GO STRAIGHT TO THE NEXT PERSON.
Selling cheap often gets things out of the door quicker than freecycling.
Ebay can be a complete pain, but then again for more expensive items it can maximise your market size and there is no hassle of dealing with people face to face... list it sell it package it post it.
Then again loads of people are so nice... I remember a teenager and his dad coming over to buy my son's old guitar... the son spent quite a long time checking it out (fair enough, instruments need to be right and it wasn;t cheap), they were so nice.