I think it depends. There have been cases rising in the US where dealers are getting in trouble for not only selling drugs, but selling drugs that have resulted in ODs. I think it's more along the lines of dealers knowingly selling something under the label of one thing, when it has something else in it.
I don't know if you know who Mac Miller is. But he was sold drug's that had fentanyl in them without his knowledge. He OD, and because of that his dealer was arrested and charged. It was a way harsher sentence because of the death, and what the drugs were laced with.
That was more of a high profile case of course though, but I think the police are more likely to look into tracing it back if the drugs are resulting in people dying.
Now, if you are simply buying a drug that could result in an OD, and not laced in anything, perhaps that's different.
Either way, I don't think you should tell a dealer what your intentions are. Do research yourself and come to the best conclusion.