I sold pot for most of my youth and never felt bad about it. Considering all the ridiculous stuff I see people do when drunk or on prescription meds, I definitely felt like my morals were at least on par with bartenders and most pharmacists.
That said, I took over a sibling's buddy's coke phone for a few days one time to pay for a trip to a wedding and holy crap did I feel terrible doing that. The money was amazing and although giving it to some people out at the bar who are craving a reckless time isn't so bad, the number of people sitting by themselves at home and asking for more and more is disgusting.
On the last night I was moving it, I went and meditated when I had some time just to calm my nerves as I was so disgusted with myself and what I was doing to people. I wound up with this horrible sense of foreboding that something bad was going to happen if I didn't listen to my conscience and quit immediately but I didn't want to piss off the guy whose phone it was and decided I'd suck it up and finish the night. Wound up getting attacked by two customers and took a hammer to the back of the head. Managed to get the car into reverse and push out the guy hanging from the car window who was trying to unlock the car door to pull me out. Back window was smash and I was gushing blood but escaped.