dealers couldnt sell opium if they wanted to, its not economical to grow a huge field of popies for one batch of opium when you can make heroin much more cost effectively.
Opium is grow to make heroin. I don't think turning Opium into Heroin has anything to do with cost efficiency, it has to do with how space is at a minimum when smuggling an illegal drug...
Why smuggle a gram of Opium, when a gram of Heroin is worth much more, since Heroin is a much more potent drug....
If your going through with the risk of smuggling/transporting an illegal drug, you have to use the available space wisely, therefore dealing in Heroin, the better potency/quantity drug makes more sense...
Since the process of turning opium into heroin is pretty simple, this is naturally the smarter option....