From my limited experience with buying cars, as well as from theoretical considerations, this won’t work because you lack the pre-commitment to buy at the price offered. Once they give you a favorable price, you can try to push it even further downwards, possibly by continuing to play the dealerships against each other. So they’ll be afraid to offer anything really favorable. (The market for new cars is a confusopoly based on concealing the information about the dealers’ exact profit margins for particular car models, which is surprisingly well-guarded insider knowledge. So once you know that a certain price is still profitable for them, it can only be a downward ratchet.)
The problem can be solved by making the process double-blind, i.e. by sending the message anonymously through a credible middleman, who communicates back anonymous offers from all dealers. (The identities of each party are revealed to the other only if the offer is accepted and an advance paid.) Interestingly, in Canada, someone has actually tried to commercialize this idea and opened a website that offers the service for $50 or so (unhaggle.com); I don’t know if something similar exists in the U.S. or other countries. (They don’t do any sort of bargaining, brokering, deal-hunting, etc. on your behalf—just the service of double-anonymous communication, along with signaling that your interest is serious because you’ve paid their fee.) From my limited observations, it works pretty well.
From my limited experience with buying cars, as well as from theoretical considerations, this won’t work because you lack the pre-commitment to buy at the price offered. Once they give you a favorable price, you can try to push it even further downwards, possibly by continuing to play the dealerships against each other. So they’ll be afraid to offer anything really favorable. (The market for new cars is a confusopoly based on concealing the information about the dealers’ exact profit margins for particular car models, which is surprisingly well-guarded insider knowledge. So once you know that a certain price is still profitable for them, it can only be a downward ratchet.)
The problem can be solved by making the process double-blind, i.e. by sending the message anonymously through a credible middleman, who communicates back anonymous offers from all dealers. (The identities of each party are revealed to the other only if the offer is accepted and an advance paid.) Interestingly, in Canada, someone has actually tried to commercialize this idea and opened a website that offers the service for $50 or so (unhaggle.com); I don’t know if something similar exists in the U.S. or other countries. (They don’t do any sort of bargaining, brokering, deal-hunting, etc. on your behalf—just the service of double-anonymous communication, along with signaling that your interest is serious because you’ve paid their fee.) From my limited observations, it works pretty well.