The key to successful negotiation - any negotiation, is to understand what is really happening in the minds of the parties. Professional negotiators know this i.e. salepeople, lawyers, politicians, diplomats, hostage negotiators etc.. The average layperson does not understand this and are like the fabled blind men encountering an elephant for the first time - they only perceive that part which is in front of them, and they have no idea of the entire animal they are standing before.
Every negotiation is different and the best strategy depends on the situation of the game and the players at that time and place. Pros know that in order to be successful, they have to know their own strength, weaknesses, motivations, limitations, but also the other party's too. There are technical aspects e.g. balance sheet, profit/loss, supply/demand, but also emotional e.g. human behavior, both rational and irrational.
Email reverse auction method works well if the supply of cars is adequate and there are many dealerships to choose from. It's not at all effective if there is limited supply and few dealerships. Some dealerships do not respond to email reverse auctions, some do. That's why you have to start with a large email blast to 10-20 dealerships. Some dealerships are more sensitive to end of month quotas. Email reverse auction works well for most people because it mitigates their biggest weaknesses: lack of skill, experience, emotional control, discipline.
Most salepeople are not emotionally invested in the negotiation. They are disciplined. It's imposed upon them because usually they have almost no freedom to make offers without the authorization of the sales manager. However, they know that the average buyer is not disciplined and overly emotional and easily manipulated. They know the average buyer does not understand or have experience or skill in negotiation. They know that the #1 motivation for the average buyer is to get the uncomfortable haggling over with quickly and buy the car for a "reasonable" price - and that what is reasonable in the buyer's mind is easily manipulated. That's how tactics like home turf advantage, anchoring, teaming, stalling, rationalizing, bait and switch, appealing to higher authority, throwing a fit etc. all work.
It's like a novice challenging a grand master at chess - beating them is near impossible if they don't want to let you win. I don't play chess. I try to get to win-win for both parties. I try to solve the puzzle of, "how can I find the hidden lowest price among all the dealerships near my location"? In order to do that I have to find the hidden lowest price at each dealership in my area, then compare them with each other. The hidden lowest price is usually not revealed by the dealership until they truly believe they are faced with "take it or leave it" situation where they believe I will exercise my best alternative to accepting his offer - buying it from another dealer. Any lower than that and the dealer will exercise their best alternative: take their chances and wait for another buyer. That's why the end of the month is such a good time. That 1 more car might fill a quota that unlocks manufacturer holdbacks or additional allocation or some other value.
Me personally, I do not like negotiating for cars in person because it's a large time commitment. I have many car dealerships close by. I like to "prime the pump" by test driving, calling and visiting the closest dealership where I'm likely to bring the car for service. Then I send out the email blast. The closest dealers I may call them again to egg them on a bit and get their competitive juices flowing. Competitive drive is an emotion that all salespeople respond to, as well as the fear of losing revenue to the enemy. Interestingly in my last 2 purchases in the last 12 months, none of the dealerships I visiting in person and did the dance with the sales team came even close to the best offer. The best offers all came from dealerships I didn't even call on the phone, only email.
For a widely available car that is not in super high demand it should be possible to check if a negotiated price is a good one. As a general rule for most cars an OTD price 10% below the OTD MSRP is a starting point for a good price. With incentives and tough negotiating 15%+ discount is not impossible. Also, look at the TrueCar price chart. The TrueCar average price is the sucker's price. A good price should be completely off the left edge of the chart.
Disclaimer: nothing written here should be taken as legal advice, but I did stay at a Holiday Inn Express last night.