I used to sell cars while going to college. I didn't do it for the money but for the experiences so that I can buy one myself. You know the tricks that they could pull on you, someone on the inside gotta learn the trade to be one up on them.
The car the I bought, I told them straight up that I used to sell cars for a living. Upon hearing that, all they wanted to do is to talk about the job and how different they are against my former boss at toyota dealership and less focus on the selling part. In and out quick. Even the finance manager did not even push to sell me them $400 floor mats either. They just gave up.

Pro tip: I look for a family with kids. They are the sure buyer. Buying a car is a whole family experiences. On a side note, because I was the youngest sales guy on the force, they pushes them "jacks" aka guys that are not commit in buying any car but just wanted a test drive onto me. I no care since I was there for the experiences and I needed the training on how to sell. I stopped pushing for a sale and just be a customer service guy with info. Amazingly, they bought from me. I had to split 50% of the credit to the other sorry azz sales guy though. It is called a hello turn, meaning that they go say hi to a customer and they would turn it to another guy to close the deal. 50/50 easy money.