Interestingly, I actually use this strategy when I make large personal purchases as well. It has always gone well. There's an element of strategy involved, however, such as buying on the last day of the quarter when sales quotas/bonuses/manufacturer bonuses, etc all come into the forefront of everyone's mind at the dealership.
I never try to put the other person in the unenviable position of actually having to ask the manager (not the fake ask the manager show they put on). I do however call up friends and find out what the thing costs on the invoice and then go in with that as my offer. (If you are diligent enough to poke around online forums and find the right people you could probably get the same information).
Success rate: The last 2 vehicles I've bought I could have sold the same day for more than I paid.
Interestingly I bought one new car once at below dealer invoice. I did all my research and then their starting offer was low.
Story turned out to be that they didn't control the mix of cars that they got, but they could sell every Prius with all options for top dollar. So internally they marked other cars down to sell fast so that they could get more Priuses.
It goes to show how dealer incentives don't always align with the manufacturer's.
There are definitely quirks to the way car dealerships are run. I imagine there are similar 'weaknesses' in just about every industry if you put in the due diligence to find them. I always find it somewhat comical when incentives backfire in interesting ways. There are a bazillion tales from HR/Recruitment/Salespeople out there about spectacularly stupid incentives, and I can't help but read every single one of them.
If you end up with a similar situation in your next vehicle purchase it might be worth writing up a blog. I know I'd read it!
Dealers and manufacturers have reacted to the wide availability of invoice pricing. Invoice is not the real price the dealer pays anymore. There are many other incentives involved, such as survey scores for luxury brands or mix of models sold (sell X of a model that isn't moving and we'll allocate you Y of the really hot model you can sell for MSRP because they're in demand).
The purpose of this system is partially to obfuscate the real "invoice" price so buyers searching the internet (or services like Edmunds) can't accurately price the car. It also allows the manufacturer to exert more control over dealers.
I never try to put the other person in the unenviable position of actually having to ask the manager (not the fake ask the manager show they put on). I do however call up friends and find out what the thing costs on the invoice and then go in with that as my offer. (If you are diligent enough to poke around online forums and find the right people you could probably get the same information).
Success rate: The last 2 vehicles I've bought I could have sold the same day for more than I paid.