My thinking? If I'm polite and verbally confirm that I'm good for the amount and the salesperson treats me poorly? Not someone I'd want to do business with anyway.
Actions >> appearance
What we do screen for a bit is basically time wasters. It is possible to be accidentally picked up in this net, and then say "they sure missed out, I bought a car at the next place". That's unfortunate for the salesperson sure, but you're ignoring the countless hours saved by quickly dismissing the true time wasters.
I sold cars and was very good at it (better than top 1%) and would sometimes get in a mood when I would ignore intuition for a few weeks and just do an amazing job with everyone. Doing an amazing job works well, and you can get people to buy a car who swear they stopped with ZERO intention of actually buying a car that month even, very often. But you almost never even get CLOSE to selling a car to someone you have a bad vibe about, even if you go all in.
If you're good at car sales, you have more people to follow up with and do a good job with than you have time. I sold 34 cars a month average over years. National average is 11. Eventually you have to trust your judgement to save yourself 10+ hours a week, even if once a month it costs you a sale.
He walked out, went to the other Ford dealer, and drove out with a new truck.
I can see not wasting time if there are many customers in the place, of course you triage them. But if you're chatting around the coffee machine because there are no customers in the showroom ... !
Unsold cars on the lot are burning cash, as they're there on borrowed money.
BTW, he had to get the manager's signoff on the final price, so I'm sure he wasn't getting fired for it. A savvy manager isn't going to let a salesman underprice a car to sell to his buddy.
You mention a lot of considerations, all perfectly valid. That's what makes the game interesting and fun :-) I'm sure the salesmen also size of the customer based on his clothes, the car he arrived in, his demeanor, etc. I remember shopping for a new car long ago, arriving at the dealer with my usual worn out jeans. None of the salesmen would talk to me. So I bought the car from the dealer's rival. These days, Seattle is full of millionaires wearing jeans, and the salesmen know that, and don't make that mistake.