In the UK, there are on-line services that serve a similar role but with the haggling done in advance. I bought my last car via www.drivethedeal.com, and wound up saving about 1/6 of the best quote I was getting from the local dealerships, which itself wasn't offensive. As far as I can tell, they did it by singling out one dealership in the entire UK that was willing to sell for very narrow, if any, margins, but then generated enough volume for that one dealership that presumably they qualified for perks from the manufacturers or similar.

I offered the local dealers the chance to match the price, but after hearing the figure I'd been quoted, they politely declined. They claimed that they would actually lose money on the sale if they did, and while I'm not an expert on the UK new car market, I suspect given the size of the discount that the local guys were being honest there.

Caution: That was mid-2000s, so any recommendation for that particular deal site is stale. Still, I have no reason to believe they have changed and apparently they're still going.