Just my two penneth:
I could have bought an ex fleet Is200 from where I work, for £3K less than a dealership. same age, just higher milage and no lexus service history.
However, I decided to go for a lexus dealership:
Just after receiving the motor (A w 2000 reg), there was vibration at braking from meduim speed: Warped brake discs. Had them replaced along with new pads under warrenty. Now, would a cheaper private sale have given you that?
Furthermore, someone just rear ended me last week (was fun seeing my lexus is various bits of disarray at the repair centre today). Got given the same car as a curteousy car whilst mine is off the road - I don't think your insurance company (unless it's that funky one off the telly) would have done that - It would of ended up being a corsa!
Anyway - Your choice, but there is a little more protection if you plump for a dealer (and I got free matts as well!).