Over the years I've bought and sold many cars and motorbikes via Auto / Bike Trader and Pistonheads. It's noteable that by far the greater percentage of sales have been to buyers who have seen the ads on Pistonheads. I've also bought most cars and bikes through P/H as well, my Phoenix Yellow came via there.
Never had a problem with either A/T or P/H - it's like everything else in life though, excercise a bit of caution, look out for scams and you'll not get suckered.