Are you saying heroin specifically because you do buy it but not from websites, or because some drugs you'd trust to a website (regardless of whether you personally buy them or not) but heroin not, or were you just naming a random drug?
And I thought it was a memorable way to phrase it... I've never seen heroin, except on The Wire.
For all drugs, it's quite possible that online offers better quality. I've known cocaine users whose local dealers only sold shitty quality, afaik I don't know anyone who has bought online but it might make sense for them in that case. And drugs like weed, might well be that you want a choice of strains whereas most dealers just have whatever they managed to buy, not a supermarket range of choices.