It's a FACT that some dealers will sell new vehicles for below cost. One very simple example is when dealers are trying to hit stair-step incentive goals near the end of the month. This is where a manufacturer will pay out a bonus incentive on EVERY vehicle sold that month, if and only if the dealer hits a certain sales goal. If they are off by 1, they don't get any bonus. If they sell that one additional vehicle, they may get as much as $100,000 in bonuses. Dealers sell cars for below cost all the time.
I'm not sure how your comment contradicts mine. If you get a $100,000 bonus for selling a vehicle, the marginal profit on that vehicle is damn near $100,000. Dealerships don't sell cars for a loss. Of course, that doesn't help the consumer unless the consumer wants that particular model, with the particular options that need to be sold to get the bonus.
Regardless, the stair-step incentives is just ONE of many examples I can provide.
The backlash against TrueCar's initial business model showed there were some dealers selling vehicles for below cost, in hopes of making up the difference with add-on sales. Whether these dealers made up the profit difference on the whole does not change the fact that at least some of those customers purchased vehicles at a profit loss to the dealer.
"[...] chances are you will get a zero-profit or even below-cost deal on your new car"
Let's say I'm a car dealer and I need to sell one more car to get my bonus. Let's say that car cost me $20,000. Let's say the bonus is $100,000. Now let's say I give away the car for free. I still have $80,000 more than I started with at the beginning of the comment.