I have never had a good experience buying from street dealers - meaning, dealers who just wander the streets openly looking for customers. Tried to buy pills quite a few times outside clubs, always got ripped off. Tried to buy weed once after a rave at like 5 AM or something from some massive black guy, got sold a bag of some stringy green shit, fuck knows what it was. My friend had the same experience another time in the same location.
It might be different with other drugs, but think about it, if these people are likely only gonna see you once and you're doing something illegal anyway, they don't have much incentive to providing customer satisfaction.
If you're gonna try anyway I'd say make sure you're with enough people that you can't be intimidated into anything, make sure whoever you're buying from probably isn't physically intimidating anyway, and make sure you're buying when the streets aren't pretty much deserted. Also don't ever buy drugs that you can't test on site, like pills. Really though there's not much point buying from anyone who you haven't been referred to through other means, IMO. Oh another thing, if the dealer is stupidly sketch about letting you look at what they're selling you that's an obvious warning sign.