I’ve bought 2 cars through carsdirect.com. You are actually buying them through local participating dealers. Carsdirect is like a broker, the prices they show will take into consideration any rebates offered, and discounts, that sometimes puts the price below invoice. You still need to go get the car at the dealership. The great thing about the internet, is now most dealerships have a internet sales department, and this is the best experience (painless) way I have been buying cars. Also, recently I noticed that carsdirect.com was not the best price. I checked with the Costco method (same thing as carsdirect with participating dealers), and they were $200 below invoice, when carsdirect was a couple hundred below retail.
In closing, it’s safe, it’s easy, and it’s the best way to go if you do not like to play games (haggle, waste time) trying to get to that same price.