I've sold a few E's back in the day, I run weed errands some times, often just get a free spliff etc. No big time dealing though, I try to keep a minimal amount only so if I do get caught, I can play off personal use.
Meyer Lansky used to do a little somethin somethin. I don't deal because I don't need to. Plus, there are other hustles to explore...such as robbing dealers at gun point. It's more profitable.
I have a couple friends who are street pharmacists. They're both pretty much addicted to the drugs they sell and spend on all they're profit on getting high.