The reason why some people get ripped off and some don't: Because some people do research and others don't. (Mechanical issues aside - And even then, take it to a private shop before buying!)
I was in car sales and now I'm running our "custom" department, but I was the exact same way with every single customer as far as my attitude and my work ethic. Some got awesome deals, and some didn't. When we shot our first offer, some people would counter, but some wouldn't.
If the price on the car is 17,995.00 and you (as sales person) think it's worth maybe 16.5...and somebody says "Sure, I'll take it" at the asking price, are you going to go back and say, "Actually, no, I won't take that because it's only worth 16.5!" No, you wouldn't. And if you did, you wouldn't be in sales very long.
Bottom line is, no matter how nasty and sneaky your salesperson could be, if you're properly prepared you'll get a good deal. (Unless they just say no, which they might.) Keep in mind, much of the buying process is also picking out a good dealership to work with. Don't just test drive the car, test out the dealership as well. I don't understand how people sign the dotted line without doing any research. If somebody came up and said "Give me $xx,xxx", wouldn't you ask a couple questions first?