actually the best site for me was lexus. i did my research on edmunds, kbb and carsdirect, but i did the actual purchase through the internet sales dept of a lexus dealer.
i've found that the best deal is going directly through internet sales. luckily for me, i live in a highly lexus dealer populated area, so i've got my pick of at least 10 dealers. i shopped from amongst 6 dealers though, since not all of them had the car i wanted.
I bought mine off ebay and must say that if I saw car prior I would not have bought that one. Sometimes dealers use online sales to get rid of cars that may not meet the standard of being on the lot. Mine had rub marks on the rear bumper and the front bumper is screwed up also. It is just not worth the trouble to fight with them being 15 hours away/
Besides my first car (did they have the internet in 1991?), I have searched for all of my cars on autotrader.com. While I would never buy a car sight unseen, I think if already have a car in mind, and you want to shop comps, and save a lot of time driving around, the internet is the way to go.
With that said, once I found the cars I was interested in, I would drive to the seller and poke around, not letting on to my prior knowledge of the specific vehicle, as it is not in my "bargaining" advantage, to let the seller know, "I have been searching the internet for weeks for a car on your lot and I want to buy it".
I find www.cars.com easier to use, as the search results are more concise, but growing up flipping through those black and white Autotrader magazines dreaming about what car I was going to buy with my first job, keeps me loyal to the Autotrader brand.
I found my IS 300 on autotrader.com at a Lexus dealership 45 miles from my home. I dealt with the sales consultants from David McDermott's Lexus of New Haven's Internet Sales Department.
The car had everything that on it I wanted including chrome wheels. I negotiated with them to Certify Pre-Own the car and install a full round of rubber.They also performed the 60K mile service as required by the CPO certification process which additionally gave me a 36 month/100,000 warranty and 4.9% APR financing.
I was very satisfied the way that I was treated at David McDermott Lexus of New Haven. They claim to be the #1 Lexus dealership for customer retention.