I think the fact that it is a black market product means the sellers know they can get away with ripping you off more than above board commodities, as you say. If people are selling weed, that already says a lot about them as they are obviously willing to break the law. It obviously depends who you know, but from my experience, the dealers i've been involved with have no respect for their customers. One guy even got me to drive ages to meet him, and then got me to drive him around places, as if I wasn't uncomfortable enough already.
And then there's another dealer who normally only sold shitty homegrown, so when they finally get some high grade in, they start charging £25 a bag...that's just not on. Another one sells high grade 20 bags at only 1.4 grams, but sells the homegrown at 2.8 :/
I stopped using the term eight a long time ago, because I don't think I have ever received 3.5grams for a 20. And on this note, I would happily pay £30 for 3.5g of good bud.