if you get poor quality gear you definitely must let the dealer know that it was poor quality and you're not happy about it. the general code of ethics (is there such a thing in the black market) in this case would be for the dealer to make up for it with the next purchase... usually by using less cutter.
dealers are in the game for two reasons (or both) and that is to make money or to sustain their own habit. either way it is a business for them and keeping customers coming back is important.
local dealer gets gear from his dealer at 30% purity, leaves a third uncut from that purity, the second third he cuts 1/1, the third 2/1.
newer customers will receive uncut gear, also the dealer may provide this to his customers who have received poor gear last time.
he will give the 1/1 gear to customers who have come back to purchase and have either said the last purchase was acceptable.
he will give the 2/1 gear to his customers who regularly are happy with the 1/1 gear and keep coming back.
if said dealer has rent payment due, or has whacked up 3 grams and only has 0.5g left to sell to repay his debt to his dealer, guess what... that 0.5g will stretch a long way for him.
smart dealers will try to make more money off you, its a business why wouldn't they? we all know stories about shonky mechanics ripping people off, add a criminal element to that and you can see what happens.