That is my constant concern exactly. It is easy to make a small batch of RU and have that send for analysis and then later mix that pure RU with fillers (harmful or not) - just like any other drug dealer. Why should sellers of "black market" RU be any different. Might be paranoid here but hate the fact that you can never be sure of what you get. It would be great if they could offer some sort of organized group buy directly which could be shipped to a 3rd party testing facility that could receive the entire batch for mixing, testing and reporting.
Agreed, the Poles also sent me a certificate about the "purity" of my product. I didn´t even read it because of the reason you gave. But the RU from Kane and other suppliers was tested numerous times by BUYERS and afaik it always turned out to be legit.