When you say "drug dealer", most people think of some gangster type who makes a lot of money by exploiting poor addicts. But that's not the reality.
If you use drugs, and you don't have a job, selling a bit of the stuff is the most obvious way to make some money. So a lot of drug users will become drug dealers as well. I think that these people need help as well; putting them in jail doesn't help anyone.
But it can get even worse. Imagine that you are a drug user, and you need some drugs. So you ask a friend, who is also a drug user, if they have any left, and if you can buy it off of them. Boom, they're a dealer now as well.
So in reality, a large part of drug users are going to be drug dealers as well. If you decriminalise drug use, but keep prison sentences for dealing, a lot will hinge on the question what makes someone a dealer.