Had similar experiences to the OP. Tried selling on Gumtree, Pistonheads, Ebay and Autotrader. Autotrader provided the most useful leads but even then they were weird, I had one guy come and view the car on behalf of someone else, I asked him if they had asked him to view the car because was knowledgeable on the subject and he told me he'd never owned a car and didn't even drive!
Also had people ringing later than 11, asking endless questions etc then offering 1/2 the asking price or a swap for a 1990 Vauxhall Nova 1.0 or something random and pointless.
I don't get why people with half the cash are even looking? Surely when buying a car the first thing people do is think "my budget is this so I can afford these" not "I want this car, maybe the seller will just take half the money"