The book _Don't Get Taken Every Time_ helped me a lot, buying my /second/ car. I was a clueless geek buying my first, and got totally suckered.
I saw sales people as clueless and useless until I joined my first startup, whereupon we /needed/ them. Our CEO hired a sales guy, all smarmy and slimy, and that head sales guy hired a team (mostly old buddies he knew), and I saw a real sales force in action. I swear they could have sold enterprise licenses to a 'Notepad' clone, it was amazing. It didn't matter to them /what/ they sold, they just sold.
Based on my many years on both sides the short answer is they tell buyers what they want to hear (hit the hot buttons regardless of whether the product or service can deliver) and they outright lie. (Really). Not that there aren't honorable good salesmen (there are) but many do the same thing that guys do to pick up women in bars.
I saw sales people as clueless and useless until I joined my first startup, whereupon we /needed/ them. Our CEO hired a sales guy, all smarmy and slimy, and that head sales guy hired a team (mostly old buddies he knew), and I saw a real sales force in action. I swear they could have sold enterprise licenses to a 'Notepad' clone, it was amazing. It didn't matter to them /what/ they sold, they just sold.