Everyone seems to think buying from a trader is awesome.
I've just helped my father replace his 530d. Found him a great candidate, we went up and viewed it, the car was great but OMG the dealer was the biggest and most irritating nightmare in the universe. If the car wasn't so good with such good provenence (07, full BMW history, serviced 2 weeks ago at local BMW dealer) we'd have walked. It was just frought with stupidity at every turn. It took an HOUR, yes an HOUR to convince the guy to let us take a test drive of more than a mile down the road and back. An hour!? He was filled with stupid comments like 'I've got this Honda and it has an electric boot close, the BMW wont do that!', seriously, if I told you all the boneheaded and stupid things this guy came out with you'd never believe me.
Added NOTHING to the purchasing process and just made the entire thing a complete hinderence from start to finish. I wish it had been a private seller as I doubt the utterly worthless warranty that came with the car is worth anything either.
I ended up just treating it like a private sale and ignoring all the dealer stuff.
Do not consider the fact a car you want is from a dealer as a plus point. In 90% of cases it simply isn't.