As a buyer I’d always look at Autotrader and ebay (and probably Pistonheads). Maybe that’s not true for everyone, but it must be increasingly likely that people are aware of both.
I quite like the fact that it’s easier to make an arms-length first contact via ebay (i.e. via email) to ask some basic questions and apply a first-level filter, whereas with AutoTrader you’re always straight in to a telephone call. It seems to be this same mechanism that protects you from canvassers on ebay – they haven’t yet resorted to emailing eBay sellers to get their phone numbers.
On the flipside AutoTrader has (in my opinion) better fitering facilities to find the car you’re looking for in the first place.
Local papers hereabouts are generally only full of tat.
As a seller I’ve sold 2 cars and a motorbike via eBay and never had any trouble with timewasters.
So, on balance, I’d also be in the ebay-first-then-Autotrader-if -necessary camp. Wherever you advertise the most important thing is to provide plenty of information up-front and be realistic about your pricing.