Just to get it out of the way, I work in sales. I think their is some intrinsical link between working as a salesperson and being perceived as one of these so called leeches.
Anyway, I work as 1 person in a 2 person team that sells family photos to schools on weekends in a large city. We were on the outskirts of the city one day and we were up to our last client before lunch. The business essentially works by the photos being taken immediately afterwards they are sold to them for a relatively cheap price compared to industry standards, there is no obligation to buy.
So during the process of me showing this lady the photos she makes it abundantly clear to me multiple times that she "[has] no interest" in looking at any individual photos of her children (as there is one free photo given to each family with the booking and the rest are additional purchases) and because of this I decided not to try and upsell the hell out of her and just let her get the free photo.
She chooses the photo, I show her what it's going to look like cropped and edited, she leaves - as does my obligation to her and her appointment.
Just as my colleague and I were leaving the venue for lunch she rushes over in a crazed panic, "YOU DIDN'T SHOW ME MY KIDS PHOTOS, THIS IS F**KING RIDICULOUS".
(At this point I know exactly what is going to happen if I don't give her exactly what she wants, she has the 'can I speak to the manager' hairstyle, you know the one)
"I understand your frustration but unfortunately we've turned off all the machines and are heading out for lunch. If you'd like I can try and be super quick and come back in about 20 minutes (we are meant to have an hour for lunch) and I can show you the photos again"
She begrudgingly agrees
20 minutes pass and I come back with my food and a 'can do' attitude
The lady then proceeds to take another 30 minutes looking at her photos, chooses to buy nothing, complains some more, makes empty threats about shutting us down because of our poor behaviour. At this point I'm just happy she's heading towards the door.
Tonight I just got an email from my boss saying how stupid the call she just had with this lady was where she claimed that I was a "blood sucking leech" who only wanted her money. My boss then said that she knows that we're much more upbeat and happy people than this lady said we were and apologised for having to put up with that.
In the end, people in sales aren't blood-sucking leeches and we'll actually go a long way to get you what you want, we are still people too.