Whatever you know you are not supposed to do as a Christian is a sin.

If you are questioning the ethics and morality of selling drugs that's probably a good indication that there is a doubt in your mind about the behavior which is probably a good indication that the behavior is a sin.

Christians according to the Bible are supposed to respect the law, and the people who enforce that law.

Selling drugs is against the law in most places. Therefore selling drugs would be against Christian doctrine at least in places where it is illegal to do that.