Generally i found AutoTrader to have less timewasters.
I used to buy and sell for a while, and i hardly got any interest from PistonHeads Classified.
Ebay is hit and miss - General rule is dont use an auction, put a BUY IT NOW/ Best Offer - and dont accept any offer on the site until they have paid a deposit/ come and paid cash (otherwise youll just spend ages relisting all the time)
There are loads of timewasters out there. - i say this after worked as a 'professional car salesman' - As well as doing it on my own as a 'Home Trader'