And nobody died in an attempted murder or was stolen in an attempted burglary but that doesn't make it any less of a crime.
If my understanding of entrapment is correct, it really doesn't matter whether narc bought or sold the drugs. What matter is whether the junkie would have taken up on the opportunity that they otherwise wouldn't have done so. It's easier to say junkie was entrapped if narc offered to sell the drugs, but it wouldn't be entrapment if junkie had approached the narc and persisted for a hit from a "reluctant" narc.
See my post about how entrapment is proved in the court.
And some more reference:
Wiki
Law.com Dictionary