the main thing isn't being cut etc it's is it what it says it is at all?
This is where tested kits and word-of-mouth help out. Not every dealer/supplier is out to screw people over with crap gear that isn't what it is supposed to be. Some actually see the value in a bit of quality control leading to repeat custom.
Back when Methadrone first started to be paraded around as MDMA/MDA/MDEA, it didn't take long for people to write off entire batches of gear after horror stories from other people.
Some people tested their gear, and wouldn't stock the fake stuff, as it would hurt repeat business.
Sure, buy off some random, you will probably get duff stuff, but how exactly does the risk of that happening add to the experience?