Me: I don't want to back and forth haggling all day. Give me the best possible price on that Grand Prix and if I like it, I'm going to buy it.

As someone who is comfortable with haggling, when people say stuff like this I instantly know they are not experienced with negotiating so I am not surprised that the salesman tried taking you for a ride. Personally I get so annoyed when people say stuff like "What's your best price?" or "Can you go any lower?"

That's not how it works, you either pay the asking price or make a counter offer. Otherwise there would be no such thing as negotiating, every 18 year old kid or busy soccer mom would just go to a salesman/vendor/dealer etc and just say "hello sir, I'd like your best price please!" and walk away with a phenomenal deal.

If you don't like haggling then don't haggle, but you'll need to look for an item that is already priced at a number you are comfortable paying. Or as a general rule if you think a price is in an okay range but you think you may be able to get a better deal just say "would you accept [10%-20% less than asking price]?" but it's up to you to know the actual approximate value of the item. Any salesman can slap any number on any item so getting even "%50" off may still be a horrible deal.