My wife works for a large dealership as a new car salesman (company fleet sales, not to the general public 89% of the time). She reckons the US marketing guru who coined the phrase "The customer is always right" should be drawn and quartered.
Once you sign the contract for the purchase of the new vehicle, that price does not change.
The vast majority of new cars sold in Australia are built overseas. With long lead times on deliveries with pretty much any manufacturer nowadays, even domestic cars, if you want that special paint job, or factory fitted GPS and they haven't got one on the showroom floor, your going to have to wait months for it to be built in an overseas or even local factory that has to schedule production based on world wide demand and consumption.
If a trade-in is offered, it should be good for 30 days, but this varies from dealership to dealership. The reason being that most valuations for second hand vehicles are based on mileage, condition etc etc.
If the lead time on delivery of your new vehicle is 3 months, a person who uses their current car for, lets say 'deliveries' could clock up 20,000 kilometers in 3 months, considerably changing the value of the car at the time the offer for a trade was made.
But your right when you say car sales people are dogdy, my wife is. She reneged on a binding agreement we had for cuddle/sexy time this morning. Just because it's Xmas morning and she is afraid the kids will get up at any minute, who cares, a deal is a deal.